Support for Nested Lists

[SergioEstevao]

Fixes #448

This PR adds support for nested lists only.
It's still not 100% but I wanted @diegoreymendez and @jleandroperez have a look on the code until now, and play around to see if they see any major issues.

I will continue to work in fixing the failing unit tests.

To test:

• Run the sample app and check if the content shows correctly
• Interact around the nested lists and see if all is working correctly
• Interact on regular lists and see if all things are still working.

[SergioEstevao]

So I fixed the unit tests by adding an option on the formatters to say if we are increasing the depth of the list or just replacing the current level.

I tested it a bit and it looks it's working pretty well on the visual side. On the DOM it looks that when you change the list type on a nested list if gets a bit confused and moves the nested list out of the <li> element. @diegoreymendez do you mind to give it a look?

[jleandroperez]

Looking good on my end!!

[SergioEstevao]

Discussed with @diegoreymendez online and decided that we can tackle the DOM issues on a later PR after a newline refactor that is on the way.