Add debugging functions.

This will run pdb, set a break point and clear all the breakpoints.

I haven't set any keybindings or implemented a history look up as it seems worthwhile to see if this is something others use. 

Let me know what should be changed / updated. Also, if this is OK to merge but you'd like to see some feature added, please feel free to give me a ticket. 

Thanks for taking a look!