Add "Share By Email" extension (#63)

This is a very old request (november 2013!) to be able to share an
article directly from FreshRSS. I don't think it should be within the
core of FRSS, so I played a bit with the extension system. I keep it
very simple for now, just to try to see what was possible with the
current system. I just needed to add a new placeholder `(~ID~)` to
FreshRSS_Share.

It relies on the mailing system introduced for the "email validation"
feature.

Reference: https://github.com/FreshRSS/FreshRSS/pull/2707

Closes https://github.com/FreshRSS/FreshRSS/issues/300

extensions.json

@@ -152,6 +152,14 @@
       "author": "CNTools | Clemens Neubauer",
       "url": "https://github.com/cn-tools/cntools_FreshRssExtensions",
       "type": "gh-subdirectory"
+    },
+    {
+      "name": "Share By Email",
+      "description": "Improve the sharing by email system.",
+      "version": 0.1,
+      "author": "Marien Fressinaud",
+      "url": "https://github.com/FreshRSS/Extensions",
+      "type": "gh-subdirectory"
     }
   ]
 }