Add Go module support

go.mod

+module git.autistici.org/ale/crawl
+
+require (
+	github.com/PuerkitoBio/goquery v1.5.0
+	github.com/PuerkitoBio/purell v0.0.0-20180310210909-975f53781597
+	github.com/PuerkitoBio/urlesc v0.0.0-20170810143723-de5bf2ad4578
+	github.com/andybalholm/cascadia v0.0.0-20181012154424-680b6a57bda4
+	github.com/golang/snappy v0.0.0-20190904063534-ff6b7dc882cf
+	github.com/pborman/uuid v0.0.0-20171128162732-e53336930665
+	github.com/syndtr/goleveldb v0.0.0-20190923125748-758128399b1d
+	golang.org/x/net v0.0.0-20190926025831-c00fd9afed17
+	golang.org/x/text v0.0.0-20190829152558-3d0f7978add9
+)