Support in-word expansion

[cbarrete]

UltiSnips supports that (link to doc), which is very convenient in some cases.

For example in LaTeX, fooref can get expanded to foo~ref{}, which is very nice since I don't want to have a space in-between.
I've tried to hack around that by having a "body": "\<BS>~\\ref{$1}", but that's not supported.

Admittedly, this is quite a niche feature, so I'd understand if you didn't want to add this to vsnip.

[hrsh7th]

It is good candidate as first implementation to support vsnip key in snippet definition, I think.

[hrsh7th]

#136
#155