TextView: Customizable default text color

[azone]

Customize default text color

To test:

1. Change textView.textColor = UIColor.darkText to textView.textColor = UIColor.red in demo
2. Run the demo

[diegoreymendez]

The problem I see with this PR is that setting the font color should probably not be disconnected from HTML.

@SergioEstevao, @jleandroperez - Do you think there's a case for letting Apps set the text color without it affecting the underlying HTML?

[SergioEstevao]

@diegoreymendez I see it as a possible Appearance configuration for the editor. Some people may want to use Aztec with a black background and white text by default.

[jleandroperez]

+1 to Sergio's point!

[diegoreymendez]

Works as described.