Adding automatic clean up of bitmaps_test directory option

[brendonbribeiro]

This is a proposal, and my first PR to the project.

This feature gives the user the option to automatically clear bitmaps_test. This can be done by passing cleanupBitmapsTestDir: true on the configuration file.

bitmaps_test will be cleared in this commands:

• test: folder is cleared before the tests run. In this case, after the test is done, only the images from the last test will be available
• approve: folder is cleared after the command
• reference: folder is cleared before the command

@garris I don't know if was better to create this function as a command.

resolves #814