Make prompt configurable (better: use ipython prompt)

[ZyX-I]

This is what I have in ipython:

. And this is bipython:

. I guess it is easy to see the difference:

• bipython version uses only one color
• bipython prompt is hard to see on dark background
• bipython prompt does not provide information about virtual env (and any other information that powerline may be configured to display, but currently is not)

. I have checked out bipython code, but did not find any standard way to configure prompt. Worse, I have not found standard way to use different colors inside the prompt, so I cannot hack it like I did with old ipython (if you check out powerline code you will see that I had to add three classes with custom __str__ of which one is proxy class with custom __add__ just to forbid old ipython to leave me with plain ASCII).