Add support for 'fit', or 'it.only'

[ColinEberhardt]

A number of other test runners have the ability to run a single test using 'fit' or 'it.only'. It would be great to have support for that.

[willemneal]

Just out of curiosity, are you having issues with filtering tests via CLI? e.g. -f file_regex -t test_regex

[jtenner]

There is a test regex and file regex flag as @willemneal points out. It would be helpful to keep as-pect as simple as possible, since it's a large codebase if possible.