Adds "Read Later" option to Share Popup bookmarklet

The current share popup bookmarklet adds external articles into TT-RSS and marks them as published. This pull request expands that behaviour to include a "Read Later" option. When Read Later is selected, the article is added to TT-RSS but instead of setting it as published, it sets it as unread. This allows TT-RSS to function as a simple read later (e.g. Instapaper, Readability) service.

The changes also include a new PluginHost hook, HOOK_SHARE_ARTICLE, which allows end users to modify how the article is handled. This can be used to, for example, pull full content from the source web page, or e-mail subscribers of the published feed, etc. (I am submitting a plugin, fc_share, to the contrib repo that integrates well here.)