Add a subcommand to create a simple test files

[Jorge Zapata]

Creating a test file can be annoying since you have to do the following:

• Have a main function that takes and object as arguments
• Return the top-level describe function in the file
• The top-level describe should take the arguments passed to main

It would be great to add a subcommand to roca that creates a template test file with all of the above saving us time when adding new tests.