Implement Markdown Shortcuts

Closes #2984, #3020
Ref #1463, #3019


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#### 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.
Weird, my editor gives me jshint errors for this kinda syntax but grunt's jshint passes just fine. Using the same jshintrc. /me scratches head