add accessibility support

[sarvarrose]

Fixes #177

Example Usage:

import { faMugSaucer } from '@fortawesome/free-solid-svg-icons/faMugSaucer'
import { FontAwesomeIcon } from '@fortawesome/react-native-fontawesome'

... react component
return {
	<FontAwesomeIcon icon={faMugSaucer} accessibilityLabel='Saucer' />
}

In web, DOM looks like:

[sarvarrose]

Open Questions:

• Should aria-hidden prop take precedence over accessible if passed?
• There are 2 other props for which support can be added:
• aria-labelledby
• aria-describedby

[phazei]

@sarvarrose

The aria props are, I can only presume, for react-native web, since they don't do anything for native. I think the native first props should have priority over mobile only ones.

@charles4221 is anyone maintaining this repo? Please have someone at FontAwesome remember this exists!

[robmadole]

I'm reviewing this now against the 1.x branch.