Support for large markdown blocks before and after functions

[fabienb4]

We need a way to add markdown blocks before and after functions docs like on meteor publish. @before and @after are nice for small things like titles or notes, but large amount of markdown will make the code painful to read.

A suggestion was to be able to include markdown files for any function requiring more details using something like:

/**
 * @summary Whatever.
 * @partial myfunction.md
 */

Although it would work, I think it will make large projects very hard to maintain, with a bunch of files here and there and their inclusions inside the project's code. See #24.

Any ideas/pull request welcome.