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/options/options.css b/src/options/options.css
index 06b8c40..7ebe04f 100644
--- a/src/options/options.css
+++ b/src/options/options.css
@@ -2,6 +2,11 @@
   padding-top: 16px;
 }
 
+main {
+  margin: auto;
+  max-width: 600px;
+}
+
 .features-link {
   position: absolute;
   top: 8px;