Add tab completion to the partest command

We can complete partest options (I've excluded some that
aren't relevant in SBT), as well as test file names.

If `--srcpath scaladoc` is included, completion of test
paths will be based on `test/scaladoc` rather than
the default `test/files`. Note that the `--srcpath` option
is currently broken via scala partest interface, this
change to scala-partest is needed to make it work:

   https://github.com/scala/scala-partest/pull/49

I've also hijacked the `--grep` option with logic
in the SBT command itself, rather than passing this
to `partest`. Just like `./bin/partest-ack`, this looks
for either test file names or regex matches within
the contents of test, check, or flag files.

I tried for some time to make the tab completion of
thousands of filenames more user friendly, but wasn't
able to get something working. Ideally, it should
only suggest to `test/files/{pos, neg, ...}` on the
first <TAB>, and then offer files on another TAB.
Files should also be offered if a full directory
has been entered. Hopefully a SBT parser guru will
step in and add some polish here.

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