Allow to create symlinks rather than copy the files for plugins installed from directory

Joomla is very unfriendly for versioning. New plugins are copied into the working tree as new files.

There is possibility to use unix feature: symlinks, and make possible to distinct the working trees for main joomla source code and plugins source code.

I.e:

$ find `pwd`/plugins/system/admin8/ -ls
  5292    4 drwxrwsr-x   2 www-data joomla       4096 paź 27 01:14 /home/dexter/src/joomla/joomla-cms/dex4er/plugins/system/admin8/
  5293    0 lrwxrwxrwx   1 www-data joomla         55 paź 27 01:14 /home/dexter/src/joomla/joomla-cms/dex4er/plugins/system/admin8/admin8.php -> /home/dexter/src/joomla/joomla-plugin-admin8/admin8.php
  5294    0 lrwxrwxrwx   1 www-data joomla         55 paź 27 01:14 /home/dexter/src/joomla/joomla-cms/dex4er/plugins/system/admin8/admin8.xml -> /home/dexter/src/joomla/joomla-plugin-admin8/admin8.xml

It means that there is a git repository in /home/dexter/src/joomla/joomla-cms/dex4er/ directory for joomla-cms
and repository in /home/dexter/src/joomla/joomla-plugin-admin8 for plugin. After modifications of plugin files I can trace them in proper repository.

This modification of joomla is not recommended for productional deployment but it is greatly helpful for development.