eslint: Enable caching

Makes re-running eslint faster on dev environments.

Change-Id: I71656d8e3f889203cd5a2ccf4a76676d2ba60f74

.gitignore

@@ -14,6 +14,7 @@ node_modules
 npm-debug.log
 rebaser/config.yaml
 rebaser/rebaser.log
+.eslintcache
 
 # OS
 .DS_Store

Gruntfile.js

@@ -339,7 +339,8 @@ module.exports = function ( grunt ) {
 		},
 		eslint: {
 			options: {
-				reportUnusedDisableDirectives: true
+				reportUnusedDisableDirectives: true,
+				cache: true
 			},
 			main: [
 				'*.js',