Added custom bash prompt, based on the 'my' theme.

This colors the arrow at the start of the second line based on the exit
code of the last command: green for 0, red for non-zero

This functionality currently fails when the 'fasd' plugin is enabled,
since it is messing with the PROMPT_COMMAND function.
Merge pull request #235 from nwinkler/nwinkler_theme

Added custom bash prompt