Add support for a custom prompt

[jjvvx]

I'm really new to Bash, but the code works just fine on my end. Tried all the advertised features on the README with a custom prompt and it works just fine.

[mchav]

Great idea. I was going to get around to this. Could you give an example of how to set and unset the prompt?

[jjvvx]

Could you give an example of how to set and unset the prompt?

It's an environment variable. You can set and unset it with export WITH_PROMPT="prompt goes here". The default would look like "%c>" where%c` is the command being evaluated.

[mchav]

Alright. Let's get this merged with the updated code base and then merge.