Add Markdown Checker with markdownlint

# Markdown Checker #
Maintainer: Charles Beynon <etothepiipower@gmail.com>

## About Markdownlint ##

[markdownlint][1] is tool to check and flag style issues in Markdown files. It can also check for Kramdown warnings. It maintained by Mark Harrison (@mivok).

## Installation Requirements ##

To install markdownlint, run:

    gem install mdl

## About Styles ##

Since not everyone writes Markdown the same way, the default style preferences may not be optimal for all users. Please read the [Markdownlint README][2] to learn about Markdownlint's use of styles.

For now you can modify the syntax_checker directly, and modify the arguments used in calling `mdl` to reflect your style needs. A future version of this checker may allow these arguments to be passed through vim configureation options.

[1]: https://github.com/mivok/markdownlint
[2]: https://github.com/mivok/markdownlint/blob/master/README.md