Add formatters/printers for Evaluations

[Craigacp]

Is your feature request related to a problem? Please describe.
Tribuo's evaluations have a build in formatted output, but this is fixed and not customisable. Currently our users can create their own output format by querying the evaluation object for the metrics they want, but we could consider adding formatters that allow users to select the metrics they want and have Tribuo automatically format them into something readable (and possibly in HTML or other formats too).

Describe alternatives you've considered
Users could continue to build their own output formatter, or use the fixed built in one.

Additional context
This was requested in #62 as part of a larger discussion. It's broken out separately here for ease of tracking.