ENH: nicely handle white space for version_bump

[CJ-Wright]

It is very possible that we will want to have whitespace in the version replacement (eg it's in an indented block (setup.py?)). Currently this white space must be hard coded into the rever.xsh file (see rever's file for an example). This is rather inelegant and could cause tons of problems. Would it be possible to keep track of this white space and maintain it upon replacement?

[scopatz]

For clarification, you mean leading whitespace? Or all whitespace?

[CJ-Wright]

I meant leading, although other would be ok too.

[scopatz]

Other would be much harder.

[CJ-Wright]

We can fix problems as people yell about them, I am good with just leading whitespace.