Configure them through "plugins=path/plugin.py, ..." in the ini file.
The paths are relative to the configuration file.

This is an almost minimal implementation and some features
are missing:

* Plugins installed through pip aren't properly supported.
* Plugins within packages aren't properly supported.
* Incremental mode doesn't invalidate cache files when
  plugins change.

Previously we sometimes normalized to Windows paths and sometimes
to Linux paths. Now switching to always use Linux paths.