Add variable helm-etags-match-part-only.

Hi Thierry,

I just added the variable  helm-etags-match-part-only to helm-tags.el

From the commit message:
"This variable will allow to control if only the tags part is matched in
helm-source-etags-select."

With the former setup it would not be possible to narrow down tags
containing double colons properly.

Cheers

Thomas