Each output format is now a self-contained plugin

- Now pev supports plugins - still primitive, but it should become something cool eventually.
- Each output format is now a self-contained plugin - they're pluggable.
- There's one thing missing to have fully functional output for HTML and XML: define a root element and output it accordingly.

I'll still rewrite a good portion of the output API, but it's working fine in this PR. For instance, the text output should be built-in.

Please, let us know of any problems you run into