Improve coverage integration

[chmp]

Changes:

• ipytest.autconfig(coverage=True) (Simplify using coverage plugin ipytest.cov #112)
• Fix branch coverage (fails at the moment)
• Rename files? ($TMP/ipykernel_.../....py -> In[20])

Todo:

• Cross-ref ipytest.config and ipytest.cov docs
• Add tests (move the current coverage test into a subdirectory to hide the .coveragerc file)
• Add a warning if shadowing a .coveragerc file? (sensible: there are many different places where to look)
• Figure out whether the config file is a good idea, as it could be confusing when hiding existing .coveragerc files