Add support for specifying test function in a file (the -k flag)

In pytest, you can specify which test functions should be called inside a file with the `-k` parameter like so: 
```
py.test tests_directory/foo.py tests_directory/bar.py -k 'test_001 or test_some_other_test'
```
It would be nice to have the same functionality here with 
```
make test F=test_foo.py K='test_001 or test_some_other_test'
```