Only add specific assets (app.css/.js) plus .DS_Store to gitignore

[danielstgt]

It could be handy to add some static assets or other vendor files in /public/css and /public/js. Sure the ideal way shouldn't be that path, but in some cases there are valid reasons.

Excluding the whole path wouldn't make that possible, so I replaced them with the app.css and app.js files according to the mix config.

Additionally I've added .DS_Store. Clearing the git cache and readding everything (e.g. a gitignore update), would add these files if they aren't ignored in the global config. Personally, I always forget that, so sometimes I end up with them in my repository. Since they don't harm in any gitignore, they're also added in this PR.