3.0 - Add plugin shell.

It symlinks / copies plugin's assets to app's webroot. Thought this might be handy to use in deployment tasks.

One issue I am having is with plugins with vendor namespace. The task first creates directory for vendor name and then proceeds to create symlink for the plugin's webroot inside it, but it fails resulting in folder being copied instead. 

If the directory for vendor name exists beforehand the symlink is created without problem. So for some reason php doesn't seem to realize the directory exists when it's created by the script itself. Using clearstatcache() doesn't help.
> I've used mod_rewrite rules on some servers where symlinking was not possible. An advantage could be that you can get "direct" access to the plugin assets without having to create the symlinks or if (grmbl) the hosting-provider doesn't allow shell access, but you are allowed to customize the virtualHost.

If you can't / don't want to create symlinks than this shell is not for you. Mod rewrite rules are beyond scope.

> OK. Should users be able to have some control over which 'strategies' are allowed (i.e. also be able to fail if symlinking fails, in stead of always falling back to copy?)

Most users won't have an issue with files being copied if symlinking fails. I don't think we should try solving everyone's problem.