Integrate into the core #71

Opening this to debate if it is time to integrate this into the core.

Pros to integrate it into the core:

• Adds support for a standard unittest feature.
• A different (arguably simpler) way to execute parametrized tests via the subtests fixture.
• Some features (pytest -v does not show the 'msg' contents in the report for subtests #6 for example) are hard to implement as plugin without further changes into the core.

Cons:

• Yet another maintenance burden to the core.

Opinions?

cc @bluetech @RonnyPfannschmidt @asottile @The-Compiler