Add go module support

go.mod

@@ -0,0 +1,6 @@
+module github.com/bwmarrin/discordgo
+
+require (
+	github.com/gorilla/websocket v1.4.0
+	golang.org/x/crypto v0.0.0-20181030102418-4d3f4d9ffa16
+)

go.sum

@@ -0,0 +1,4 @@
+github.com/gorilla/websocket v1.4.0 h1:WDFjx/TMzVgy9VdMMQi2K2Emtwi2QcUQsztZ/zLaH/Q=
+github.com/gorilla/websocket v1.4.0/go.mod h1:E7qHFY5m1UJ88s3WnNqhKjPHQ0heANvMoAMk2YaljkQ=
+golang.org/x/crypto v0.0.0-20181030102418-4d3f4d9ffa16 h1:y6ce7gCWtnH+m3dCjzQ1PCuwl28DDIc3VNnvY29DlIA=
+golang.org/x/crypto v0.0.0-20181030102418-4d3f4d9ffa16/go.mod h1:6SG95UA2DQfeDnfUPMdvaQW0Q7yPrPDi9nlGo2tz2b4=