command line option for link suffix and prefix.

[tyler-gilbert]

I use Hugo to generate the website and it drops the '.md' in the links.

A command line option could be used to replace '.md' with whatever is wanted.

I also need a link prefix to get everything to work correctly.

Implemented here:

https://github.com/tyler-gilbert/moxygen/blob/master/src/helpers.js

I can create a pull request if you like.