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/bxmpp.go

@@ -0,0 +1,11 @@
+// +build !noxmpp
+
+package bridgemap
+
+import (
+	bxmpp "github.com/42wim/matterbridge/bridge/xmpp"
+)
+
+func init() {
+	FullMap["xmpp"] = bxmpp.New
+}