gitconfig: additions and tweaks

* stash aliases;
* add -A becomes add .
* push.default mode set to simple;
* rh will reset softly;
* reset HEAD alias;
* commit squash HEAD alias;
* clean aliases.

Signed-off-by: Adrian-Tudor Panescu <adrian@panescu.com>
Merge branch 'master' of github.com:adrianp/dotfiles

Signed-off-by: Adrian-Tudor Panescu <adrian@panescu.com>