Plugin system draft + example plugin

Very rough work in progress. This (will) add a plugin system for page elements. Do not merge.

The example plugin adds a `readityourself` link for each Shaarli link, pointing to a readable version of the page ([example](http://www.readityourself.net/readityourself.php?url=https%3A%2F%2Fprojects.eff.org%2F~barlow%2FDeclaration-Final.html)).
The [readityourself](https://github.com/memiks/readityourself) server URL is configurable.
Requested in https://github.com/sebsauvage/Shaarli/issues/66. To test it, `git clone https://github.com/nodiscc/Shaarli shaarli-test; cd shaarli-test; git checkout readityourself`

![howitlooks](https://cloud.githubusercontent.com/assets/3950390/4929487/b373d0b4-6553-11e4-8493-8f67151a8982.png)


#### TODO - PLUGIN SYSTEM
 * all options should be in options.php, + https://github.com/shaarli/Shaarli/issues/41
 * `LINKLIST_PLUGINS` should be an array
  * It's a list of enabled plugins that will get included/run when raintpl generates the linklist
  * adaptable to `PAGEHEADER_PLUGINS`, `EDITLINK_PLUGINS`...
 * remove the new `if` condition line 51, instead:
  * [loop](http://www.raintpl.com/Documentation/Documentation-for-web-designers/RainTPL-Tags/_loop_%20array%20___loop_/) over all plugin names in the arry, for each, [include](http://www.raintpl.com/Documentation/Documentation-for-web-designers/RainTPL-Tags/_include_%20template%20_/) `inc/plugins/$pluginname/$pluginname.html` (find a correct location).
   * the `<span...` print in the example plugin should be in `inc/plugins/readityourself/readityourself.html`
 * test test test
 * add more example plugins (archive.org ? qr code?)
 * test test
 * rebase and squash commits

This method works for most feature requests in https://github.com/shaarli/Shaarli/issues/14, and allows moving some current core items (thumbnails, qr) to plugins. Third party plugins can be cloned in `plugins/` and simply enabled in options.php.

Current status: draft draft draft. Give it some time or submit changes to https://github.com/nodiscc/Shaarli on branch readityourself