Add vspec support

[rbong]

vspec is a vim testing tool.

It uses vim syntax with additional builtin commands (describe, it, before after, context).

This adds basic support for vspec - that is, by using any of the additional builtin commands vspec will not give an error.
Since this uses parse_cmd_common it doesn't actually build the structure of the new blocks. It just treats them as normal commands.

I have only added basic support because:

• I would like your feedback on adding vspec support before I do anything else
• This is good enough for me, but I will add better support if needed

Related issue: Vimjas/vint#330

[rbong]

I realize that I'm going to have to make this smarter because :end is already used. Let me know your thoughts on vspec support and I will work more on this.