Add support for experiments

This change refactors the options logic and adds support for
experiments: a new type of options which are not shown in the options
page (their usage will be similar to Chrome flags).

Experiments can be set from the
chrome-extension://{extension_id}/options/experiments.html page.

This code refactoring simplifies the options definition. Each option now
has a default value, and a context: the place where the option is set
(options, experiments, internal, deprecated).

Change-Id: I358ae07c832acae6b4536788c4dbe12a0e4730bf
diff --git a/src/_locales/en/messages.json b/src/_locales/en/messages.json
index f3b71d3..7c8b45b 100644
--- a/src/_locales/en/messages.json
+++ b/src/_locales/en/messages.json
@@ -127,6 +127,14 @@
     "message": "Saved",
     "description": "Message which appears in the options page when the settings are saved"
+  "options_experiments_title": {
+    "message": "Experiments",
+    "description": "Title of the experiments page: a page where highly experimental options can be set."
+  },
+  "options_experiments_description": {
+    "message": "Welchrome! Below you'll a find a list of experiments: features which are in development, and are not quite ready for launch. They are highly experimental and so might break and/or cause issues, but if you're brave, please do enable the ones you're most interested in, and <a href=''>give feedback</a>!",
+    "description": "Description shown in the beginning of the experiments page, below the title."
+  },
   "inject_links": {
     "message": "Links",
     "description": "Heading which we use before the 'previous post' link in a user profile in TW"