Allow skipping of sourceDirectory or testSourceDirectory

[camac]

Fixes #31

Adds 2 extra configuration options controlling whether to skip relevant
directories:

• skipSourceDirectory
• skipTestSourceDirectory

Tests added:

• check_skipsourcedirectory: Tests that sourceDirectory is skipped
  for check goal when skipSourceDirectory is true.
  sourceDirectory includes unformatted code so it would fail if not
  skipped.
• check_skiptestsourcedirectory: Tests that testSourceDirectory is
  skipped for check goal when skipTestSourceDirectory is true.
  testSourceDirectory includes unformatted code so it would
  fail if not skipped.
• skipsourcedirectory: Tests that sourceDirectory files are not formatted
  with format goal when skipSourceDirectory is true.
• skiptestsourcedirectory: Tests that testSourceDirectory files are not
  formatted with format goal when skipTestSourceDirectory is true.

[klaraward]

Thanks @camac :)

Could you also add invoker.properties files for these tests? Might not be the most interesting thing to test via the invoker test mechanism but still nice to keep things consistent.

[camac]

Thanks @klaraward, I have added invoker.properties, switched to using loadMojo, and update the groupId in the test pom.xml's to be com.spotify.fmt