Disable monitors more flexibly

[Graeme A Stewart]

Although we implemented a mechanism to disable specific monitors from the command line (#178) it doesn't help much if a lower part of the workflow is the one in charge of launching prmon, e.g., the transform infrastructure in Athena.

For these cases it would be nice to also support disabling monitors "from the outside". This could be done by looking for an enviroment variable that would trigger disabling one or more monitiors. Then users can set this in their environment and subsequent invocations of prmon will respect it.