Add support for build tags (#1054)

By default all bridges are available.

You can turn off certain bridges by providing
e.g. "nodiscord" as a build tag.

    go build -tags nomsteams,noapi

gateway/bridgemap/bslack.go

@@ -0,0 +1,13 @@
+// +build !noslack
+
+package bridgemap
+
+import (
+	bslack "github.com/42wim/matterbridge/bridge/slack"
+)
+
+func init() {
+	FullMap["slack-legacy"] = bslack.NewLegacy
+	FullMap["slack"] = bslack.New
+	UserTypingSupport["slack"] = struct{}{}
+}