Link external URLs

[Kikobeats]

Sometimes a need to reference a external URL from the sidebar.

For do that, basically I add this markup:

* [JS MythBusters](/README.md)
        * [Send your tip!](/send-tip.md)

when send-tip.md is just a empty file necessary for be compiled by docpress, and later using JS I change the href of the URL generated.

Could be nice support URL's directly from Docpress.

[rstacruz]

Yes, this seems like a great addition. It'd be nice to directly link to GitHub, StackOverflow, Hexdocs.pm, and other externals.

[kzap]

does it already work directly on external markdown files? sometimes i need to reference MD files from other repositories, but then if they are private that would be an issue...