Add Python script to parse shader assembly

At the moment it basically does the same thing as Shader Converter (but
also does ps_2_0, doesn't do *s_1_*, handles sincos and shouldn't
randomly put dots after output declarations) and automatically comments
out any preshaders.

My intention is to add more smarts to this over time, which is why I put
more effort into tokenising and parsing the assembly rather then fuzzy
regexp matching like I usually would.

Definitely needs more testing on a wider variety of shaders.