Add font-awesome icon selection to alarm markdown sugar

[kakposoe]

Hi there!

I've been tinkering around with some functionality to add the ability to use font-awesome icons to alarm markdown sugars.

Below is an example of new functionality to be added with this pull request:

> {danger.fa-exclamation-triangle} This is dangerous

> {success.fa-check} This worked

Alert sugars will now check to see the alarm type and if a font awesome icon have been declared using a dot notation (e.g {success.fa-check}).

Note: As this plugin currently uses font awesome 4.7.0, a suggestion would be made to upgrade to use a later version (>=5.0.0) to avoid confusion.

There is also a small refactor of the default svg icons.

[kakposoe]

Another change would be need to update to a later version of font awesome as the class definitions has changed (e.g .far .fa-check).

[saleem-hadad]

This is awesome ❤️
thank you very much @kakposoe