Create the very first bits of a TuneIn Radio plugin for Volumio.
So far it simply downloads and shows the list of available categories
from TuneIn Radio.
Everything else is still to be done.

Add initial (and very unclean) support for playing a radio.
The "Local Radio" section now supports playing radios correctly.

Significant changes in the browsing logic, including:
* Load root categories during the onStart() call, and cache results
  for a faster browsing
* Use regexp matches to identify the 1st level of nested browsing
* Generalise the former browseLocal() function to be able to handle
  most of L1 browsing, covering Local radios, Talk, Music and Sport
* Fix the "prev" link

Start migrating the codebase to use node-tunein-radio instead
of node-tunein, which is being designed with the specific
purpose of supporting the required use-cases for this
Volumio plugin.

The browsing logic had to be reviewed after switching to the new
node-tunein-radio npm module.
All main categories are now working when it comes to browsing their
main content.

It looks like `albumart` needs to be used for thumbnais instead of
`icon`, which is used for folder icons instead.

Implement search through TuneIn content

* Add support for 2 meta categories (Best and Popular) via configuration switches.
* Break down items in each list by sub-categories

Track the history of browsing across different nested levels in
order to provide a more consistent behavior when pushing the
"back" button.

* Use the parsed URL from API calls to set internal routing URLs, which
  makes the whole logic much more generic

* De-duplicate code by moving common bits into a function that will assemble
  the proper item to be added to the result sets

Since TuineIn uses an undefined number of nested levels with filters
and so on, we need to take a generi approach while browsing for
content, and trust the results from API without trying to
over interpret.

The API uses uppercase characters and the ~ sign is some filters
(Namely when paginating by first letter).

* Make the existing entries in UIConfig more self-explanatory.
* Add a new flag for enabling/disabling experimental features to
  make sure people are clear on what they get.
* Hide "Podcasts" behind the experimental flag.

Leverage the ability of the node-tunein-radio module to cache results.
Given that the contend doesn't change so often, cache results for
one hour.

Now it's possible to browse Podcasts / Shows episodes and play
them properly.