Allow dynamic colors to enable e.g. Red prompt symbol '$' on errors

[eugenesvk]

I'd like to have a red $ prompt symbol when the last operation status was an interruption/error (e.g. $? is False), this PR allows me to do that:

• via passing a function that outputs Red or Green (depending on the last op status) in the theme fg color field
• via adding a $saved_lastoperationstatus=$? variable in your prompt function to allo using this status in the function above

[lukesampson]

Cool, thanks