Feature/named capturing groups

[siburb]

Added two new properties to Style to allow us to iterate through the named capturing groups in the regex, and also to retrieve styles by name.

This allows us to apply any other style to a named sub-range of the matched range.

Added an example for h3 to the 'one-dark-custom' style.

#28

[siburb]

Whilst this works, it doesn't allow the named groups to inherit the font/traits/size of the parent. Hold off on reviewing it for now and I'll do some work on that.

[ruddfawcett]

This looks great thus far — thanks, @siburb.