Refactor markdown parsing of Image and ExplicitLink

[tompng]

Create intermediate rule ExplicitLinkWithLabel that returns {label:, link:}, so that Image rule does not need to re-parse rdoc link format again.

# Before
"[text](url)" → "{text}(url)"
"![text](url)" → Image("{text}(url)") →(parse with regexp)→ "rdoc-image:url:text"

# After
"[text](url)" → ExplicitLink({ label: 'text', link: 'url' }) → "{text}(url)"
"![text](url)" → Image({ label: 'text', link: 'url' }) → "rdoc-image:url:text"

#1322

[ioquatix]

Thanks!