Correct link shortcut so that it inserts and highlights correctly in the editor.

Closes #2614
- When no text is selected insert link markdown and move cursor to a into the [] portion.
- When text is selected insert link markdown and move selection to http:// text so proper link can be pasted right away.
Merge branch 'shiftctrll' of https://github.com/shindakun/Ghost into shiftctrll