Write BEL Style guide (BEP8)

[cthoyt]

I've already started writing an opinionated style guide here: https://github.com/pharmacome/curation/blob/master/style-guide.rst

While some of it may be a bit picky, we should have some minimum community standards for BEL scripts. Example: is there a preferred maximum line length? Is it written anywhere that spacing is important?

@ncatlett I think we would best write this together