Extra context: API + CLI

Fixes #12
Synthesizes parts of: #75, #100, #116, #161 and #199.
Thanks @fcurella, @aventurella, @emonty, @schacki, @ryanolson!
I insisted on keeping the test before because I wanted to be certain that the behavior of cookiecutter remained the same. So it wasn't a test of `stdlib` but rather the difference between `stdlib` and `unicode_open.

Now that 0.7.2 is gone this test can be removed. However, I submit that it's removal doesn't belong in this pull request. Rather it's own pull request.