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/experiments_bit.js b/src/options/experiments_bit.js
new file mode 100644
index 0000000..af8e086
--- /dev/null
+++ b/src/options/experiments_bit.js
@@ -0,0 +1 @@
+window.CONTEXT = 'experiments';