add emoji parsing support to gfm markdown

A simple implementation to work with emoji's like proposed in original #233 issue.

Unlike the complex idea mentioned in #233 this is a more elegant solution that only generates the replacement html code (or whatever you override as template). So no complicated data modules or funky stuff. 

Users will need to source and host the images for themselves, but can customise the template function to be as as simple or fancy as they require (examples provided).

Closes #233, work to solve https://github.com/isaacs/npm-www/issues/392
What about instead of `true`, passing in a string will be used as the directory (and if you want more fine-grained control, you can use a function)? I imagine almost nobody will use `/graphics/emojis` as the directory, and defaults like this seem out of place.