Add Medium output and push support

[jmmv]

Medium now provides an API to publish posts programmatically, and what's better is that they support both Markdown and HTML as input:

https://github.com/Medium/medium-api-docs#32-posts

https://medium.com/developers/accepted-markup-for-medium-s-publishing-api-a4367010924e#.6mbsup39t

It would be great to have the ability to publish posts directly to Medium from Markdown2Social. Because our input files are Markdown, we might not need to do any conversions on them before they are pushed, but maybe we need to do some tweaks to get good-looking posts. Still to be investigated.