Add flag for document uri xpath to docs2harvest.

Summary:
 - an xpath can be specified to select the document URI from
 the document.

Test Plan:
./bin/docs2harvest test/data/*
./bin/docs2harvest --no-data test/data/*
./bin/docs2harvest --document-uri-xpath //head/title --no-data test/data/*

Reviewers: radu.hambasan

Reviewed By: radu.hambasan

Differential Revision: http://phab.code4fun.de/D54