Allow vimdoc to crawl non-standard directories

[Soares]

Plugins with directories outside of the standard autoload/ftplugin/etc. may want those custom directories to be noticed by vimdoc. That's not currently possible.

[dbarnett]

Rather than adding a new vimdoc directive and creating potentially confusing scenarios where one .vim file pulls in another, one option would be to create a vimdoc.cfg or vimdoc.json file with global vimdoc config. Some other global config such as @order could be migrated into the same global config file.