Add an option to set a CSS class to be applied for each link type

[miscoined]

Thanks for making this addon! I thought I'd add some functionality that I've been wanting. This does two things:

• Apply the linkified class to linkified links both in live preview and reading mode
• Add a field for a custom CSS class to be applied for each link pattern, defaulting to nothing.

The main point of this pull request is the last one, but I thought it was nicer/more convenient to apply the same class in both modes so that custom snippets can target both with one selector.

[matthewhchan]

Thanks for the contribution.