Change the beautifier to conform to the expectations of elm-format

src/beautifiers/elm-format.coffee

@@ -1,9 +1,12 @@
 ###
 Requires https://github.com/avh4/elm-format
 ###
-
 "use strict"
 Beautifier = require('./beautifier')
+Promise = require("bluebird")
+fs = require("fs")
+readFile = Promise.promisify(fs.readFile)
+rename = Promise.promisify(fs.rename)
 
 module.exports = class ElmFormat extends Beautifier
   name: "elm-format"
@@ -13,6 +16,14 @@ module.exports = class ElmFormat extends Beautifier
   }
 
   beautify: (text, language, options) ->
+    tempfile = @tempFile("input", text)
+    .then (name) =>
+      newName = name + ".elm"
+      rename name, newName
+      .then () =>
         @run("elm-format", [
-      @tempFile("input", text)
+          '--yes',
+          newName
           ])
+        .then () =>
+          readFile newName, {encoding: 'utf-8'}