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/os_file.go

@@ -0,0 +1,93 @@
+package stdlib
+
+import (
+	"os"
+
+	"github.com/d5/tengo/objects"
+)
+
+func makeOSFile(file *os.File) *objects.ImmutableMap {
+	return &objects.ImmutableMap{
+		Value: map[string]objects.Object{
+			// chdir() => true/error
+			"chdir": &objects.UserFunction{Name: "chdir", Value: FuncARE(file.Chdir)}, //
+			// chown(uid int, gid int) => true/error
+			"chown": &objects.UserFunction{Name: "chown", Value: FuncAIIRE(file.Chown)}, //
+			// close() => error
+			"close": &objects.UserFunction{Name: "close", Value: FuncARE(file.Close)}, //
+			// name() => string
+			"name": &objects.UserFunction{Name: "name", Value: FuncARS(file.Name)}, //
+			// readdirnames(n int) => array(string)/error
+			"readdirnames": &objects.UserFunction{Name: "readdirnames", Value: FuncAIRSsE(file.Readdirnames)}, //
+			// sync() => error
+			"sync": &objects.UserFunction{Name: "sync", Value: FuncARE(file.Sync)}, //
+			// write(bytes) => int/error
+			"write": &objects.UserFunction{Name: "write", Value: FuncAYRIE(file.Write)}, //
+			// write(string) => int/error
+			"write_string": &objects.UserFunction{Name: "write_string", Value: FuncASRIE(file.WriteString)}, //
+			// read(bytes) => int/error
+			"read": &objects.UserFunction{Name: "read", Value: FuncAYRIE(file.Read)}, //
+			// chmod(mode int) => error
+			"chmod": &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(),
+						}
+					}
+
+					return wrapError(file.Chmod(os.FileMode(i1))), nil
+				},
+			},
+			// seek(offset int, whence int) => int/error
+			"seek": &objects.UserFunction{
+				Value: func(args ...objects.Object) (ret objects.Object, err error) {
+					if len(args) != 2 {
+						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(),
+						}
+					}
+					i2, ok := objects.ToInt(args[1])
+					if !ok {
+						return nil, objects.ErrInvalidArgumentType{
+							Name:     "second",
+							Expected: "int(compatible)",
+							Found:    args[1].TypeName(),
+						}
+					}
+
+					res, err := file.Seek(i1, i2)
+					if err != nil {
+						return wrapError(err), nil
+					}
+
+					return &objects.Int{Value: res}, nil
+				},
+			},
+			// stat() => imap(fileinfo)/error
+			"stat": &objects.UserFunction{
+				Value: func(args ...objects.Object) (ret objects.Object, err error) {
+					if len(args) != 0 {
+						return nil, objects.ErrWrongNumArguments
+					}
+
+					return osStat(&objects.String{Value: file.Name()})
+				},
+			},
+		},
+	}
+}