Don't destroy selection/cursor anchors when encompassed by a change

Add the 'strong' option to anchors. If anchors are 'strong' instead of being destroyed by encompassing changes they move to the beginning of the change range.

src/app/anchor.coffee

@@ -9,14 +9,17 @@ class Anchor
   screenPosition: null
 
   constructor: (@editSession, options = {}) ->
-    { @ignoreEqual } = options
+    { @ignoreEqual, @strong } = options
 
   handleBufferChange: (e) ->
     { oldRange, newRange } = e
     position = @getBufferPosition()
 
     if oldRange.containsPoint(position, exclusive: true)
-      @destroy()
+      if @strong
+        @setBufferPosition(oldRange.start)
+      else
+        @destroy()
       return
 
     if @ignoreEqual

src/app/cursor.coffee

@@ -12,7 +12,7 @@ class Cursor
   wordRegex: /(\w+)|([^\w\s]+)/g
 
   constructor: ({@editSession, screenPosition, bufferPosition}) ->
-    @anchor = @editSession.addAnchor()
+    @anchor = @editSession.addAnchor(strong: true)
     @anchor.on 'change-screen-position', (args...) => @trigger 'change-screen-position', args...
     @setScreenPosition(screenPosition) if screenPosition
     @setBufferPosition(bufferPosition) if bufferPosition

src/app/selection.coffee

@@ -226,7 +226,7 @@ class Selection
       @trigger 'change-screen-range', newScreenRange unless oldScreenRange.isEqual(newScreenRange)
 
   placeAnchor: ->
-    @anchor = @editSession.addAnchor()
+    @anchor = @editSession.addAnchor(strong: true)
     @anchor.setScreenPosition(@cursor.getScreenPosition())
     @anchor.on 'change-screen-position.selection', => @trigger 'change-screen-range'