Spacemacs support

[hjorthjort]

New features:

• Documentation how to add support for inline highlighting in Markdown for vanilla Emacs.
• Automatic support for inline highlighting in Markdown for Spacemacs.
• Several updates to the syntax highlighting, including highlighting cells, rewrites, and matching logic.
• Configurability, letting the user specify extra keywords they would like to be highlighted.
• Adds Marmalade package format to k-mode.el for MELPA support, as described in Marmalade package format #6

This also replaces the current k-mode.el with the new one. As far as I can tell, the existing k-mode.el is a very slight variation of the legacy k3-mode.el. I think we are better off using this new k-mode.el for both vanilla Emacs and Spacemacs, using additional customization options for users who want personalized highlighting.

[hjorthjort]

@milseman Are you okay with these changes? Do you want to remain listed as an author? And would you be alright with us licensing this under a GPL Free Software License, so it can be added to MELPA?

[hjorthjort]

@ehildenb or @dwightguth Could you merge this? I'm trying to reach @milseman about the GPL license, but haven't heard back yet. But it should not be a blocker for this.