Handle `--append` for vim/emacs, composition approach

[mislav]

Implements --append for Vim & Emacs formatters:

# vim:
ripper-tags -a foo.rb bar.rb
# emacs:
ripper-tags -a -f TAGS foo.rb bar.rb

The implementation of --append differs from Universal-ctags in the way that, instead of merely appending new tags from listed source files to existing tag file, the old tags that belonged to listed source files are removed. This ensures that there are no outdated tags in the final tags file.

This implementation differs from #84 in a way that it takes the composition approach via new formatter classes, rather than extending VimFormatter/EmacsFormatter with extra functionality. This is done to minimize the potential for bugs introduced to non-append mode.

Closes #84, fixes #49

[mislav]

Credit: @k3rni for initial spike in #84. 🙇

[k3rni]

Excellent!