Add yas-snippet-trim feature

[conao3]

Hi!
First, thanks for such a awesome package!

I add snippet-trim feature and add its optional variable named
yas-snippet-trim.

This feature is disabled at default, but if the user enable,
string-trim snippet before expand snippet.

This is a great solution for #767, #365, #192.
And this issue is also written in FAQ

[conao3]

FYI: @holocronweaver, @edgimar, @joaotavora