Fixes #169

* Open social media in new tab and add Medium

svg file pulled from Medium post on their logos
and brand guides dated Aug 22, 2017

https://medium.design/logos-and-brand-guidelines-f1a01a733592

* Add new-window accessibility icon for social media icons

Visible on-hover of link-transition class, this PR also
adds a description to the social media icons indicating that
they will open in a new window.

This way, users will be able to see and hear (via a reader)
when a tab will open in a new window.

This approach is based on the one outlined here:
https://medium.com/@svinkle/why-let-someone-know-when-a-link-opens-a-new-window-8699d20ed3b1

2.0 Enhancements

- Improve template structure
- Fixed issue where flexbox was configured improperly and the side column expanded with longer content.
- No longer assume that posts are called "post"
- Simplify tag handling
- Improve related posts handling
- Support native Table of contents (if declared in params)
- Add DISQUS commenting (if disqusShortname added to config)
- Uses Hugo's Related content feature 
- Add asset hashing to Webpack for performance