Support image alt text for accessibility

[fiznool]

trafficstars

The current implementation does not correctly parse alt text from an image, as the markdown-it parser does not appear to support this out of the box.

As taken from the markdown-it renderer source, this PR adds the alt text attribute so it can be used by the image render rule. The default rule passes this through as an accessibilityLabel, so it can be read by VoiceOver.

[iamacup]

Merged into react-native-markdown-display here: https://github.com/iamacup/react-native-markdown-display/pull/13