Add 'scalafmt' fixer for Scala files

closes https://github.com/w0rp/ale/issues/1299

doc/ale-scala.txt

@@ -2,6 +2,37 @@
 ALE Scala Integration                                       *ale-scala-options*
 
 
+===============================================================================
+scalafmt                                                   *ale-scala-scalafmt*
+
+If Nailgun is used, override `g:ale_scala_scalafmt_executable` like so: >
+  let g:ale_scala_scalafmt_executable = 'ng'
+
+To disable `scalafmt` globally, use |g:ale_fixers| like so: >
+  let g:ale_fixers = {'scala': []}
+<
+
+See |g:ale_fixers| for more information on disabling fixers.
+
+
+g:ale_scala_scalafmt_executable               *g:ale_scala_scalafmt_executable*
+                                              *b:ale_scala_scalafmt_executable*
+  Type: |String|
+  Default: `'scalafmt'`
+
+  Override the invoked `scalafmt` binary. This is useful for running `scalafmt`
+  with Nailgun.
+
+
+g:ale_scala_scalafmt_options                     *g:ale_scala_scalafmt_options*
+                                                 *b:ale_scala_scalafmt_options*
+  Type: |String|
+  Default: `''`
+
+  A string containing additional options to pass to `'scalafmt'`, or
+  `'ng scalafmt'` if Nailgun is used.
+
+
 ===============================================================================
 scalastyle                                               *ale-scala-scalastyle*