yasnippet.rcp must now download submodules

Yasnippet was recently changed to use a submodule for its `snippets/` dir, that is, it no longer bundles snippets directly.

Therefore, to sucesfully install yasnippet *and* a default snippet collection, el-get must download its submodules.

The only downside to the proposed change it that it will also recursively download another, possibly non-essential submodule: the yasmate tool (github.com/capitaomorte/yasmate) and along with it more related textmate bundles, also implemented as submodules.