src/plugin.c: Move the perl and python "global" magic to plugin_load().

The config file handling is really not the best place to have this. Also,
the feature I'm working on would have to copy this magic if left in the
config code, and I *really* don't want that.
src/plugin.c: Fix a comment regarding duplicate plugin_load() calls.
src/configfile.c: Add the "AutoLoadPlugin" option.

When enabled, <Plugin ...> blocks will automatically load plugins.
Thanks to Tim Bunce for suggesting this!

Github: #333