.bashrc: Add bash_completion for Mac OS X

Will allow you to tab complete stuff like ssh remote paths etc once
you install it via brew:

        brew install bash-completion

Signed-off-by: Alan Orth <alan.orth@gmail.com>

.bashrc

@@ -8,6 +8,10 @@
 # OS-specific things
 if [[ "$OSTYPE" =~ ^darwin.*$ ]]; then
     PATH=$(brew --prefix coreutils)/libexec/gnubin:$PATH
+
+    if [ -f $(brew --prefix)/etc/bash_completion ]; then
+        . $(brew --prefix)/etc/bash_completion
+    fi
 else
     alias less='less -R' # preserves colors in `less`
 fi