Center the screen on the tag after jumping.

This also makes the behavior consistent with buffertag, changes, and
quickfix.  It originally used `sil! norm! zvzz`, just as the others do,
but that didn't play well with the use of `feedkeys()` in
`ctrlp#tag#accept()`.  Instead, we now use `feedkeys()` to do the
centering.
Merge pull request #53 from jszakmeister/center-tag-after-jump

Center the screen on the tag after jumping.