Correct link shortcut so that it inserts, selects text or moves cursor properly

 Closes #2614
 - Determine proper place for selection, or simply move the cursor.
 - When no text is selected insert link markdown and move cursor to a into the [].
 - When text is selected insert link markdown and move selection to http:// so URL can be pasted.
Merge pull request #2629 from shindakun/shiftctrll

 Correct link shortcut so that it inserts, selects text or moves cursor properly