rewrite command line handlers to use a Reporter class

[willkg]

We need a Reporter class that reports in specific formats: cmdline, JSON, HTML, singleline (for SublimeText, Emacs, etc), etc.

The user would be able to specify the reporter to use with a command line argument.

First step is to write the Reporter class and the CmdLineReporter. We can push off other reporters to different issues.

Things to think about:

1. reporters should be plugin-friendly
2. how do we handle options being passed to the reporter? like indent=4, pretty=true, etc?

[willkg]

This is related to #32 and probably needs to be done first.

[willkg]

This is also related to #23 and probably needs to be done first.