Feature: add ^U prefix to allow changing buffer name, like M-x shell does

[shentonfreude]

i've got a bunch of Pipenv shell buffers and don't know which is which. It would be handy to name them when you create them, like I can do with ^U M-x shell. (I tend to name my shell buffers for the dir I'm in working in to help me remember).

Thanks for pipenv.el.

[pwalsh]

#45