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/ca/messages.json b/src/_locales/ca/messages.json
index c4c74cc..2c3ed92 100644
--- a/src/_locales/ca/messages.json
+++ b/src/_locales/ca/messages.json
@@ -127,6 +127,14 @@
     "message": "Desat",
     "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": "<i>Welchrome!</i> Aquí a sota trobaràs una llista d'experiments: funcions que estan en desenvolupament i que encara no estan llestes del tot per ser llençades. Són altament experimentals i podrien trencar-se i/o causar problemes, però si ets valent/a, sisplau activa les que més t'interessin i <a href=''>envia feedback</a>!",
+    "description": "Description shown in the beginning of the experiments page, below the title."
+  },
   "inject_links": {
     "message": "Enllaços",
     "description": "Heading which we use before the 'previous post' link in a user profile in TW"