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/bridgemap.go

@@ -2,47 +2,9 @@ package bridgemap
 
 import (
 	"github.com/42wim/matterbridge/bridge"
-	"github.com/42wim/matterbridge/bridge/api"
-	bdiscord "github.com/42wim/matterbridge/bridge/discord"
-	bgitter "github.com/42wim/matterbridge/bridge/gitter"
-	birc "github.com/42wim/matterbridge/bridge/irc"
-	bkeybase "github.com/42wim/matterbridge/bridge/keybase"
-	bmatrix "github.com/42wim/matterbridge/bridge/matrix"
-	bmattermost "github.com/42wim/matterbridge/bridge/mattermost"
-	bmsteams "github.com/42wim/matterbridge/bridge/msteams"
-	brocketchat "github.com/42wim/matterbridge/bridge/rocketchat"
-	bslack "github.com/42wim/matterbridge/bridge/slack"
-	bsshchat "github.com/42wim/matterbridge/bridge/sshchat"
-	bsteam "github.com/42wim/matterbridge/bridge/steam"
-	btelegram "github.com/42wim/matterbridge/bridge/telegram"
-	bwhatsapp "github.com/42wim/matterbridge/bridge/whatsapp"
-	bxmpp "github.com/42wim/matterbridge/bridge/xmpp"
-	bzulip "github.com/42wim/matterbridge/bridge/zulip"
 )
 
 var (
-	FullMap = map[string]bridge.Factory{
-		"api":          api.New,
-		"discord":      bdiscord.New,
-		"gitter":       bgitter.New,
-		"irc":          birc.New,
-		"mattermost":   bmattermost.New,
-		"matrix":       bmatrix.New,
-		"rocketchat":   brocketchat.New,
-		"slack-legacy": bslack.NewLegacy,
-		"slack":        bslack.New,
-		"sshchat":      bsshchat.New,
-		"steam":        bsteam.New,
-		"telegram":     btelegram.New,
-		"whatsapp":     bwhatsapp.New,
-		"xmpp":         bxmpp.New,
-		"zulip":        bzulip.New,
-		"keybase":      bkeybase.New,
-		"msteams":      bmsteams.New,
-	}
-
-	UserTypingSupport = map[string]struct{}{
-		"slack":   {},
-		"discord": {},
-	}
+	FullMap           = map[string]bridge.Factory{}
+	UserTypingSupport = map[string]struct{}{}
 )