Add the ability to run individual monitors via pytest?

[jwhite3]

It might be useful to add the ability to execute individual monitors or custom groups of monitors with the monitor runner (and therefore pytest).

Currently, the only way to execute an individual monitor is to either use the API in IPython or write a script to execute it. Being able to take advantage of the pytest machinery for custom groups or individual cases would be great for consistency in allowing for command-line use