Use grc to make bash less boring

[`grc` (Generic Colourizer)][grc homepage] is a simple tool which
brings colour to boring Bash commands.

Read the full [`grc` README][grc README] mirror repository on GitHub

Check out the [`grc` mirror repository on GitHub][grc github].

[grc homepage]: http://korpus.juls.savba.sk/~garabik/software/grc.html
[grc README]: http://korpus.juls.savba.sk/~garabik/software/grc/README.txt
[grc github]: https://github.com/pengwynn/grc

I've done the following changes:
- Install `grc` via `brew`
- Copy `$(brew --prefix grc)/etc/grc.bashrc` to `~/grc.bashrc`.
  `grc` colourize the `diff` command output by default. I've removed that
  since there already is a `diff()` function in `.functions` which uses `git`.
- Source the `grc.bashrc` file in `.bash_profile`.