Wiremad Preview support for Markdown

Adds Wiremad wireframing supporting to markdown preview files.

Usage

Create wireframes in markdown as follows:

```wiremad
# Login Page
[[ Logo | Sign In | Help ]]
## Welcome Back
Sign in to your account
[Email address___________]
[Password_______________]
[ ] Remember me
[Sign In] Forgot password?
---
Don't have an account? [Sign Up]
```` 

Wireframes will only be rendered in preview (Ctrl + Shift + V)

Syntax

Syntax guide is here: https://akonan.github.io/wiremd/guide/syntax.