Revert "Config: Added plugin paths."

The plug-in path doesn't belong into the configuration file. It is
installation-dependent, not project-dependent. Therefore the better
alternative would be to specify it via a command-line argument which
can be supplied by an installation-specific wrapper script.

This reverts commit 05760d5872fa1d47d710447ef50ad6c2ded84c53.
Test: Pass plug-in path to Zwobot.

All existing tests pass an empty plug-in path. So far it is ignored by
Zwobot.
Search for plugins in provided plugin path.

Added the concept of an plug-in path that is searched for plug-ins.
This enables built-in plug-ins, in addition to those specified within
the project.
Refactoring.

Created function stderr that encapsulates all prints to os.Stderr.
Refactoring.

Created function stdout that encapsulates all access to os.Stdout.
Added time to output.

This makes it easier to tell, if any output has been printed while you
were looking at another virtual desktop.
Refactoring.

Improved test descriptions.
Added testutils.WriteFile().
testutil.WriteFile() will create directory.

If necessary, WriteFile() will create the directory before attempting to
add a file to it.
Fixed: Relative path could shadow absolute path.

Zwobot was erroneously interpreting absolute plugin paths as relative
paths. This had no consequences under most circumstances, but could lead
to a situation where a plug-in relative to the pluginPath could shadow a
plug-in specified by an absolute path. Example:

Specified absolute path: /path/to/plugin
Shadowing relative path: /pluginPath/path/to/plugin

This has been fixed and can no longer occur.
Refactoring.

Cleanup in test code.