Remove sigil for mono compat

At this time Sigil does not work on mono. 

Ideally I can also fix Sigil for mono compat, but until then it is easier to move Veil forward by removing the dependency on Sigil.

I have kept Sigil's api intact so it shouldn't be too hard to bring it back in when it works where we need it.

Sigil / Mono status: 
https://github.com/kevin-montrose/Sigil/issues/16