Add scripting (tengo) support for every incoming message (#731)

TengoModifyMessage allows you to specify the location of a tengo (https://github.com/d5/tengo/) script.
This script will receive every incoming message and can be used to modify the Username and the Text of that message.
The script will have the following global variables:
to modify: msgUsername and msgText
to read: msgChannel and msgAccount

The script is reloaded on every message, so you can modify the script on the fly.

Example script can be found in https://github.com/42wim/matterbridge/tree/master/gateway/bench.tengo
and https://github.com/42wim/matterbridge/tree/master/contrib/example.tengo

The example below will check if the text contains blah and if so, it'll replace the text and the username of that message.
text := import("text")
if text.re_match("blah",msgText) {
    msgText="replaced by this"
    msgUsername="fakeuser"
}

More information about tengo on: https://github.com/d5/tengo/blob/master/docs/tutorial.md and
https://github.com/d5/tengo/blob/master/docs/stdlib.md

vendor/github.com/d5/tengo/stdlib/rand.go

@@ -0,0 +1,99 @@
+package stdlib
+
+import (
+	"math/rand"
+
+	"github.com/d5/tengo/objects"
+)
+
+var randModule = map[string]objects.Object{
+	"int":        &objects.UserFunction{Name: "int", Value: FuncARI64(rand.Int63)},
+	"float":      &objects.UserFunction{Name: "float", Value: FuncARF(rand.Float64)},
+	"intn":       &objects.UserFunction{Name: "intn", Value: FuncAI64RI64(rand.Int63n)},
+	"exp_float":  &objects.UserFunction{Name: "exp_float", Value: FuncARF(rand.ExpFloat64)},
+	"norm_float": &objects.UserFunction{Name: "norm_float", Value: FuncARF(rand.NormFloat64)},
+	"perm":       &objects.UserFunction{Name: "perm", Value: FuncAIRIs(rand.Perm)},
+	"seed":       &objects.UserFunction{Name: "seed", Value: FuncAI64R(rand.Seed)},
+	"read": &objects.UserFunction{
+		Value: func(args ...objects.Object) (ret objects.Object, err error) {
+			if len(args) != 1 {
+				return nil, objects.ErrWrongNumArguments
+			}
+
+			y1, ok := args[0].(*objects.Bytes)
+			if !ok {
+				return nil, objects.ErrInvalidArgumentType{
+					Name:     "first",
+					Expected: "bytes",
+					Found:    args[0].TypeName(),
+				}
+			}
+
+			res, err := rand.Read(y1.Value)
+			if err != nil {
+				ret = wrapError(err)
+				return
+			}
+
+			return &objects.Int{Value: int64(res)}, nil
+		},
+	},
+	"rand": &objects.UserFunction{
+		Value: func(args ...objects.Object) (ret objects.Object, err error) {
+			if len(args) != 1 {
+				return nil, objects.ErrWrongNumArguments
+			}
+
+			i1, ok := objects.ToInt64(args[0])
+			if !ok {
+				return nil, objects.ErrInvalidArgumentType{
+					Name:     "first",
+					Expected: "int(compatible)",
+					Found:    args[0].TypeName(),
+				}
+			}
+
+			src := rand.NewSource(i1)
+
+			return randRand(rand.New(src)), nil
+		},
+	},
+}
+
+func randRand(r *rand.Rand) *objects.ImmutableMap {
+	return &objects.ImmutableMap{
+		Value: map[string]objects.Object{
+			"int":        &objects.UserFunction{Name: "int", Value: FuncARI64(r.Int63)},
+			"float":      &objects.UserFunction{Name: "float", Value: FuncARF(r.Float64)},
+			"intn":       &objects.UserFunction{Name: "intn", Value: FuncAI64RI64(r.Int63n)},
+			"exp_float":  &objects.UserFunction{Name: "exp_float", Value: FuncARF(r.ExpFloat64)},
+			"norm_float": &objects.UserFunction{Name: "norm_float", Value: FuncARF(r.NormFloat64)},
+			"perm":       &objects.UserFunction{Name: "perm", Value: FuncAIRIs(r.Perm)},
+			"seed":       &objects.UserFunction{Name: "seed", Value: FuncAI64R(r.Seed)},
+			"read": &objects.UserFunction{
+				Value: func(args ...objects.Object) (ret objects.Object, err error) {
+					if len(args) != 1 {
+						return nil, objects.ErrWrongNumArguments
+					}
+
+					y1, ok := args[0].(*objects.Bytes)
+					if !ok {
+						return nil, objects.ErrInvalidArgumentType{
+							Name:     "first",
+							Expected: "bytes",
+							Found:    args[0].TypeName(),
+						}
+					}
+
+					res, err := r.Read(y1.Value)
+					if err != nil {
+						ret = wrapError(err)
+						return
+					}
+
+					return &objects.Int{Value: int64(res)}, nil
+				},
+			},
+		},
+	}
+}