Add a shortcut for approving test output

[mjambon]

Here's how I review a test right now:

$ ./test status -s 8bdf63abd0d3 -l

If I'm happy with the output, I have to run the following command:

$ ./test approve -s 8bdf63abd0d3

It would be faster if I could just append a + to the status command that I typed earlier:

./test status -s 8bdf63abd0d3 -l +

Proposal 1: a + argument in the status command will approve the selected tests after printing their status as usual.

Proposal 2: same as proposal 1 but asks for confirmation. The user needs to enter y or n to approve all the selected tests that need approval.

Proposal 3: same as proposal 1 but asks for confirmation for each selected test that needs approval.

Proposal 4 (similar to proposal 3): a + argument in the status command causes, after printing the status of a test, to ask the user whether they want to approve this test before moving on to the next test.

See also #38.