add execContext-option to enable context-execution for completion

[xi-lef]

Resolves #333.
I mainly used code that was removed in b0ab744ab73b599b24276d87f3d80f37cf76f0e8 and added documentation and the option snippets.execContext. Its default value is false, so the default behavior of coc-snippets doesn't change.

If you're not interested in this, that's fine, but I find it very useful. Either way, I left a comment on #333, so if other people also like this, they could always just use my fork.