Differentiate built-in functions from operators. Add ban_url, regsuball and hash_data functions while we're at it

See https://www.varnish-software.com/static/book/VCL_Basics.html#vcl-functions for official list of built-in functions
Merge pull request #3 from quintonparker/master

Differentiate built-in functions from operators. Add ban_url, regsuball ...