Add system-wide keyboard shortcuts for Markdown

**tl;dr** Perform common Markdown-formatting tasks anywhere that you're editing text: Inside a GitHub comment in your browser. In a Markdown file inside your editor. In your email client. Anywhere.

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As an extensive user of Markdown, I find myself wanting to quickly perform various Markdown formatting tasks just about anywhere that I'm editing text. While many editors and Markdown-centric apps provide keyboard shortcuts for Markdown formatting, those shortcuts don't do me any good when I'm editing text elsewhere. Like right now. In this text field. For this pull request description. Inside my browser. The keyboard shortcuts from \<Atom/Sublime/Emacs/Byword/etc.> aren't available here. Cue Markdown Mode:

![karabiner-screenshot](https://cloud.githubusercontent.com/assets/2988/3639159/be99c9dc-106f-11e4-89e4-183f2a67673b.png)

#### Demo

![markdown-mode-demo-48-colors](https://cloud.githubusercontent.com/assets/2988/3639347/d3982668-1078-11e4-9fbc-ea54442a6b13.gif)

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To take Markdown Mode for a spin, just follow the setup steps in the README.