bashrc: Correction: Export variables

Export variables to the environment of the subsequent commands instead
of just defining them in the bash session's namespace.
bashrc: Activate colors in the OS X terminal

In Fedora & Ubuntu the terminal has colors by default, but in OS X the
terminal is monochrome.
bashrc: Correction: 'vimx' is only available in Fedora

Use 'vimx' as the defalut editor if it's available; use 'vim' otherwise.
packages: Add bash-completion

It is pre-installed with Linux but not with OS X.