texteditor, preview and file menu

added basic ui with texteditor, web preview and file menu.
Added Sundown library

Added Sundown (the markdown processing library) from
https://github.com/vmg/sundown/
Add markdown parsing

Add C++ interface for sundown
Parse user input in text editor and output html in webview
Fix crash when text editor is empty

Only try to render as HTML when text editor is not empty
Preview as dock widget and support different CSS

Moved HTML preview to dock widget
Support different CSS styles for preview
File actions and HTML source view

First implementation for file actions
Added tab widget and HTML source view
Add edit menu and use QKeySequence
Implement undo/redo menu actions
add custom QPlainTextEdit subclass

use subclass of QPlainTextEdit to add special features for the markdown editor widget
use dockwidget for "html source" view

use dockwidget for "html source" view and tabify it together with the "html preview"
Add key shortcut (Ctrl + Left Arrow) to remove header mark (#)
Merge branch 'feature-customeditor' into develop
Merge branch 'feature-menu' into develop
Add more CSS styles for HTML preview
redesign UI and add style menu

* removed HTML source view
* remove dock widgets
* put editor and HTML preview in a splitter
* add style menu for CSS styles
remove border around content
add line number area to editor

add a line number area widget to the markdown text editor
activate some markdown extensions
fix code style
add submodule discount (https://github.com/Orc/discount.git)