.gitconfig: Remove `push.default`

This was causing issues on servers that don’t have an up-to-date Git installation. As a workaround, I decided to remove this from `.gitconfig` and add this to `~/.extra` for my laptop only:

    git config --global push.default simple
.gitconfig: Tweak `diff` colors
init: Use the Solarized Dark theme in Sublime Text
.hgrc
- Added username

.bash_prompt
- added prompt for HG
Merge commit 'f44b1c50f696d019ad4be5d3f3cf81d4964e9bb1'

* commit 'f44b1c50f696d019ad4be5d3f3cf81d4964e9bb1':
  init: Use the Solarized Dark theme in Sublime Text
  .gitconfig: Tweak `diff` colors
  .gitconfig: Remove `push.default`