Add EXAMPLE, SOURCE, and CODE blocks

[balupton]

<!-- EXAMPLE[./example.js] --> will output the file in a syntax code block, and replaces require('./') with require('the-package-name') - useful for including example code files

<!-- CODE[./input.js] --> will output the file in a syntax highlighted code block - useful for including code files

<!-- SOURCE[./output.md] --> will output the file as is, no code block wrapper - useful for including markdown and HTML files

[balupton]

/cc @mcollina - this is the feature I was telling you about, will do it within the next 1-4 weeks

[balupton]

Just published up https://github.com/bevry/ropo which can be a foundation for this

Details at https://twitter.com/balupton/status/1006070544871985153

[balupton]

Live code running services like asciinema, scrimba, and a bunch of others seem to do this better