Add configs for whether to auto execute tests.

Add two config options to specify whether tests should get automatically
executed during build or watch. This is handy if you just want
mimosa-testem-require to handle the configuration and then manually
handle running your test suite.
This issue is an example of such a case:
https://github.com/dbashford/mimosa-testem-simple/issues/3
Merge pull request #7 from bpugh/autorun_config

Add configs for whether to auto execute tests.