Add 'editor:add-selection-below' command

It still needs work, but the basic idea is for every selection to
add another another selection over the same column range of the line
below.

src/app/edit-session.coffee

@@ -761,6 +761,9 @@ class EditSession
   selectLine: ->
     @expandSelectionsForward (selection) => selection.selectLine()
 
+  addSelectionBelow: ->
+    @expandSelectionsForward (selection) => selection.addSelectionBelow()
+
   transpose: ->
     @mutateSelectedText (selection) =>
       if selection.isEmpty()

src/app/editor.coffee

@@ -122,6 +122,7 @@ class Editor extends View
       'editor:select-to-beginning-of-line': @selectToBeginningOfLine
       'editor:select-to-end-of-word': @selectToEndOfWord
       'editor:select-to-beginning-of-word': @selectToBeginningOfWord
+      'editor:add-selection-below': @addSelectionBelow
       'editor:select-line': @selectLine
       'editor:transpose': @transpose
       'editor:upper-case': @upperCase
@@ -211,6 +212,7 @@ class Editor extends View
   selectAll: -> @activeEditSession.selectAll()
   selectToBeginningOfLine: -> @activeEditSession.selectToBeginningOfLine()
   selectToEndOfLine: -> @activeEditSession.selectToEndOfLine()
+  addSelectionBelow: -> @activeEditSession.addSelectionBelow()
   selectToBeginningOfWord: -> @activeEditSession.selectToBeginningOfWord()
   selectToEndOfWord: -> @activeEditSession.selectToEndOfWord()
   selectWord: -> @activeEditSession.selectWord()

src/app/keymaps/editor.cson

@@ -9,6 +9,7 @@
   'ctrl-]': 'editor:unfold-current-row'
   'ctrl-{': 'editor:fold-all'
   'ctrl-}': 'editor:unfold-all'
+  'ctrl-shift-down': 'editor:add-selection-below'
   'alt-meta-ctrl-f': 'editor:fold-selection'
   'shift-tab': 'editor:outdent-selected-rows'
   'meta-[': 'editor:outdent-selected-rows'

src/app/selection.coffee

@@ -148,6 +148,12 @@ class Selection
   selectToEndOfWord: ->
     @modifySelection => @cursor.moveToEndOfWord()
 
+  addSelectionBelow: ->
+    range = @getBufferRange().copy()
+    range.start.row++
+    range.end.row++
+    @editSession.addSelectionForBufferRange(range)
+
   insertText: (text, options={}) ->
     oldBufferRange = @getBufferRange()
     @editSession.destroyFoldsContainingBufferRow(oldBufferRange.end.row)