.functions: Improve `targz`

* Only use `zopfli` when it’s available and if the `.tar` file is smaller than 50 MB.
* Use `pigz` instead of `gzip` when it’s available.
* Always exclude `.DS_Store` files when creating a tarball.

Closes #199.
bootstrap.sh: Don’t make the home directory world-readable

Fixes #159.
.brew: Install native apps through homebrew-cask

Closes #200.
Merge branch 'master' of git://github.com/mathiasbynens/dotfiles

* 'master' of git://github.com/mathiasbynens/dotfiles:
  .brew: Install native apps through homebrew-cask
  bootstrap.sh: Don’t make the home directory world-readable
  .functions: Improve `targz`