Plugin helper updates.

plugin.php can now be used to load .php, .js, and .css files:

plugin.php?plugin=PLUGINNAME&page=somepage.php
plugin.php?plugin=PLUGINNAME&page=somepage.php&nopage=1
plugin.php?plugin=PLUGINNAME&file=somefile.js&nopage=1
plugin.php?plugin=PLUGINNAME&file=somefile.css&nopage=1

Subdirs are supported as well for these, so you can keep your
js and css in their respective subdirs.

plugin.php has also been modified to automatically insert
tags to load any js/* and css/* files found in the plugin
directory when it is run in 'page' mode (ie, no 'nopage').