Implement Markdown Shortcuts

Closes #2984, #3020
Ref #1463, #3019

 #### Shortcut Values
Shortcut values can now be either strings (as before) or objects like
{action:string, options:hash}.
 #### Markdown shortcuts added
- 'ctrl+alt+u' strkethrough
- 'ctrl+alt+1' h1
- 'ctrl+alt+2' h2
- 'ctrl+alt+3' h3
- 'ctrl+alt+4' h4
- 'ctrl+alt+5' h5
- 'ctrl+alt+6' h6
- 'ctrl+shift+i' image
- 'ctrl/command+k' link
- 'ctrl+l' list

Left behind commented out stubs for additional markdown shortcuts.

 #### Editor Controllers (New and Edit)
- Moved shared init function into editor-base-controller.
- Removed MarkerManager mixin from editor controllers as it's included in
  the base.
Merge pull request #3022 from novaugust/codemirror-shortcuts

Implement Markdown Shortcuts