Changed plugin pages generation concept

* the plugin listing (left side navigation bar) is now being generated
  from all plugins, available in master branch
* if the plugin had different page in previous Geany versions, these
  pages are also being generated. Links to them are included in the
  bottom of plugin page
* fixed checking exceptions in EXCLUDE_PLUGINS
Merge pull request #3 from vmkononenko/master

Changed plugin pages generation concept