Change cross-spawn dependency to be optionalDependency

Closes #573, #795, #922, #858, #925, #761, #902, #740.

If cross-spawn fails to install, atom-beautify will still continue to
install and work without it, with degraded support for Windows, such as
ignoring PATHEXT.

package.json

@@ -88,7 +88,6 @@
     "coffee-fmt": "0.11.0",
     "coffee-formatter": "^0.1.2",
     "coffee-script": "^1.10.0",
-    "cross-spawn": "^2.0.0",
     "csscomb": "^3.1.7",
     "diff": "^2.1.0",
     "editorconfig": "^0.12.2",
@@ -117,6 +116,9 @@
     "winston": "^1.0.0",
     "yaml-front-matter": "^3.2.3"
   },
+  "optionalDependencies": {
+    "cross-spawn": "^2.0.0"
+  },
   "activationCommands": {
     "atom-workspace": [
       "atom-beautify:help-debug-editor",

src/beautifiers/beautifier.coffee

@@ -2,9 +2,14 @@ Promise = require("bluebird")
 _ = require('lodash')
 fs = require("fs")
 temp = require("temp").track()
-spawn = require('cross-spawn')
 readFile = Promise.promisify(fs.readFile)
 which = require('which')
+# Get optional dependency cross-spawn
+spawn = null
+try
+  spawn = require('cross-spawn')
+catch err
+  spawn = require('child_process').spawn
 
 module.exports = class Beautifier