fix: reporters can be configured individually

The `subdir` option previously introduced in karma-coverage 0.2.5 brings a
regression: it wasn't possible anymore to configure a specific output
directory/subdirectory for each reporter. This fix makes it possible again.

Fix #91
Merge pull request #92 from karma-runner/fix-individual-dir-subdir

fix: reporters can be configured individually
Merge in latest changes from master