Refactor monitoring to "plugins"

[graeme-a-stewart]

As was noted in #2 the structure of prmon makes it difficult to easily add or remove parts of the monitoring.

I think this could be refactored along the following lines:

• Each piece of the monitoring is a class instance (cpumon, netmon, iomon, ...)
• Each of these classes implements an monitoring interface that provides the standard components need by prmon (e.g., print text file headers, get statistics, print JSON entries)

Then prmon can keep a vector of the enabled instances and just loop over them to assemble the requisite monitoring in a flexible way.

[graeme-a-stewart]

Fixed in #37, although at the moment no runtime switching of monitoring modules is done, it's much easier to support this now.