Implement a more complex vendored file detecting

The improved detecting method allows to examine if a file vendored by
not only its name but also its content.
Add Bootstrap library to vendor_complex.yml
Add test coverage for Bootstrap files
Update vendor-related section in README
Merge branch 'patch-2' of https://github.com/upsuper/linguist into mergepullrequests

Conflicts:
	test/test_blob.rb