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*
 

doc/ale.txt

@@ -218,6 +218,7 @@ CONTENTS                                                         *ale-contents*
     sass..................................|ale-sass-options|
       stylelint...........................|ale-sass-stylelint|
     scala.................................|ale-scala-options|
+      scalafmt............................|ale-scala-scalafmt|
       scalastyle..........................|ale-scala-scalastyle|
     scss..................................|ale-scss-options|
       prettier............................|ale-scss-prettier|
@@ -388,7 +389,7 @@ Notes:
 * Rust: `cargo`!!, `rls`, `rustc` (see |ale-integration-rust|), `rustfmt`
 * SASS: `sass-lint`, `stylelint`
 * SCSS: `prettier`, `sass-lint`, `scss-lint`, `stylelint`
-* Scala: `fsc`, `scalac`, `scalastyle`
+* Scala: `fsc`, `scalac`, `scalafmt`, `scalastyle`
 * Slim: `slim-lint`
 * SML: `smlnj`
 * Solidity: `solhint`, `solium`