Document extractor service (#15665)

* Document extractor service

* Fixing vendor modules

* Addressing PR Review comments

* Some small simplifications

* Fixing a linter complain

* simplifying a bit the code using package variables

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

vendor/github.com/andybalholm/cascadia/go.mod

@@ -0,0 +1,5 @@
+module github.com/andybalholm/cascadia
+
+require golang.org/x/net v0.0.0-20180218175443-cbe0f9307d01
+
+go 1.13