chore(gitignore): add .agignore

I use a .agignore file to skip the build directory and some other files
while searching using Ag (see
https://github.com/ggreer/the_silver_searcher).

Author: chirayuk

.gitignore

@@ -11,3 +11,4 @@ angular.js.tmproj
 /components/
 angular.xcodeproj
 .idea
+.agignore