Add an experimental/advanced section in options

[jchang504]

As discussed in the 07/02/17 meeting, this would provide a mechanism for developers to add interesting/experimental features with less general appeal without changing the standard behavior of KeepTabs too rapidly or bloating the options page. It should be discreet, like a small grey link at the bottom of the options that expands a bunch of checkboxes for advanced features. These would set a simple shared flag in the background script (but declared in shared_utils), which would also send a message to the content scripts to update their copy of the flag (similar to changing hold key) to enable the feature.

This is another push in the direction of looking into ES6 modules to keep the experimental code clearly separate from the main code, both for background and content script.