add haskell_disable_TH configuration variable for Template Haskell disabling

[alx741]

I found out that while writing Yesod apps the Template Haskell and QuasiQuotes syntax coloring gets flatten out like strings, and by disabling them I get nice braces and types coloring, so using:

let g:haskell_disable_TH = 1 disables the TH syntax, which would be enabled by default.

This might be a bit subjective, so feel free to dismiss it.

[raichoo]

Thanks a lot, I'll merge this.