Allow non-freelinking links in Markdown syntax to pass through to Markdown without an error

Currently, it's not possible to use Freelinking's Markdown link syntax and interoperate with the Markdown filter, as Freelinking replaces any external links (e.g. "[Example](http://example.com)") with an error message. The attached patch handles this by ignoring a link and passing it through unchanged if and only if the markdown syntax is selected, and the URL scheme doesn't match any plugin indicators, or an explicit relative link (using a leading '/', './' or '../') is used. Thus, the link falls through to Markdown, which can then handle it as a regular Markdown link.