Preserve prompt on backspace

The entire prompt must be passed to Readline otherwise Readline it
doesn't know the length of the prompt and so when you delete something
on the command line the entire prompt it's deleted (also the history
gets corrupted somehow). This was not a behavioural bug, all worked ok
but aesthetically was quite annoying
Merge pull request #118 from gabrielelana/preserve-prompt

Preserve prompt on backspace