Zsh: clean up aliases.

This commit also introduces '{session}!' commands which start/attach to
a tmux session with the given name.
Git: set preferred push behaviour for Git 1.8.
Vim: update PHP 'die' snippet.
Vim: allow syntax highlighting up to the 2048th character.

The value was previously too low for certain files.
Vim: completeopt-=longest too many issues with it.

Use shortest common completion instead of the longest one.
Vim: clean up bundles.

Drop bundles that got zero usage in the last couple of months.
Vim: +xmlformat abbreviation in command-mode.