Run peru with `python -m peru` (python -m peru.main still works too)

This helps recover from "My Python 2&3 got mixed up on Windows!"
in the least surprising way.  I'm already used to `py -3 -m pip ...`
but I never thought to check for some longer module-name
to run for peru.

I'm not aware of any downside to having this file.
Please feel free to prove me wrong. :)

Fix #176

peru/__main__.py

@@ -0,0 +1,3 @@
+from .main import main
+main()
+