.gitconfig: Add alias to merge GitHub pull request

Ref. https://help.github.com/articles/checking-out-pull-requests-locally

Closes #422.
Brewfile: add some CTF tools

See https://github.com/ctfs/write-ups for more info.
Merge branch 'master' of https://github.com/mathiasbynens/dotfiles

# By Cătălin Mariș (1) and Mathias Bynens (1)
# Via Mathias Bynens
* 'master' of https://github.com/mathiasbynens/dotfiles:
  Brewfile: add some CTF tools
  .gitconfig: Add alias to merge GitHub pull request