Make atomic leading whitespace optional (defaults to enabled)

Closes #3174

src/config-schema.coffee

@@ -155,6 +155,10 @@ module.exports =
         type: 'boolean'
         default: true
         description: 'Show line numbers in the editor\'s gutter.'
+      atomicSoftTabs:
+        type: 'boolean'
+        default: true
+        description: 'Skip over tab-length runs of leading whitespace when moving the cursor.'
       autoIndent:
         type: 'boolean'
         default: true

src/text-editor.coffee

@@ -278,6 +278,7 @@ class TextEditor extends Model
       invisibles: @getInvisibles(),
       softWrapColumn: @getSoftWrapColumn(),
       showIndentGuides: @config.get('editor.showIndentGuide', scope: @getRootScopeDescriptor()),
+      atomicSoftTabs: @config.get('editor.atomicSoftTabs', scope: @getRootScopeDescriptor()),
       tabLength: @getTabLength(),
       ratioForCharacter: @ratioForCharacter.bind(this),
       isWrapBoundary: isWrapBoundary,