Suggestion: extend the link syntax to allow DOIs

[gavinsimpson]

Currently, we need to use the Rd markup/macro to include DOIs in documentation: \doi{doi:XXXXXXXXX}. When writing roxygen using the markdown support I find it a little jarring to have to have to revert to Rd markup.

My suggestion is to expand the markdown link syntax to allow DOI links to be written. I am thinking of:

[doi::doi_string] → \doi{doi:doi_string}

For example: [doi::10.1016/j.csda.2012.01.026] would render to \doi{doi:10.1016/j.csda.2012.01.026}.