replace Filename by vim_snippets#Filename and document it

It doesn't make sense that all snippet engines have to
keep this function somewhere
ruby snippets:

use do dov for both engines
drop #TODO comments

changing defaults is bad, but I expect ruby folks to expect do to type
do ..

doo has the advantage to get a completion popup .. but its too long to
type

The UtiSnips do snippet didn't make sense: the nested placeholder still
didn't remove the space after the "do" when deleting it - so it was
written badly. Could be fixed, by why bother?

Thus learn do and dov and be happy forever. Would Do be a better choice
(upper case chars?) Don't know. This was used by UltiSnips.

Comment on this commit if you don't like this choice.