feat: Added IsEnabled support with example CustomSwitch

[chrispeabody]

For issue: #7

This branch:

• adds support for the IsEnabled property.
• adds an example CustomSwitch which uses the IsEnabled property.

[chrispeabody]

@IeuanWalker for visibility

[IeuanWalker]

@chrispeabody thanks, ill take a look when i can.

I believe there might be some platform code to prevent keyboard and voice overs activating it, But will test when i get a chance

[chrispeabody]

@chrispeabody thanks, ill take a look when i can.

I believe there might be some platform code to prevent keyboard and voice overs activating it, But will test when i get a chance

Excellent, thank you!

[IeuanWalker]

Thanks, @chrispeabody, android was fine for accessibility, but iOS needed a small fix, you can see that here #15