fix: Update asset_map to handle assets more than one level deep

[joshuabremerdexcom]

When a CSS files is used in the index.html that has a path like such: /assets/dir1/dir2/dir3/library.css. The asset map throws an exception that it can't be found. This is because the DirectoryAssetMap only looks at children, and not the entire tree. This now returns all the files found recursively for it to check. In addition, it returns the relative path, so there isn't a risk of two files in different directories colliding.

Addresses issue: #24

[joshuabremerdexcom]

I did go through the contributing guide and ran bin/setup. I couldn't get my full test suite setup, but I didn't get those specific files to pass tests. Any guidance? I can add it to my PR.

 Failure/Error: EmberCli[name].build

     ActionView::Template::Error:
       No `ember-cli` executable found for `my-app`.

       Install it:

           $ cd /Users/jbremer/repos/ember-cli-rails-assets/spec/dummy/my-app
           $ yarn install
           ```
           
           
           ```
           
 ❯ tree spec/dummy/my-app
spec/dummy/my-app
├── node_modules
│   └── ember-cli-rails-addon
│       ├── CHANGELOG.md
│       ├── README.md
│       ├── app
│       │   └── initializers
│       │       └── ember-cli-rails-addon-csrf.js
│       ├── index.js
│       ├── package.json
│       └── tests
│           └── addon-test.js
├── package.json
├── tmp
└── yarn.lock

[Samsinite]

@joshuabremerdexcom I noticed that this same section has an error if it is a non-local asset (such as google fonts) as well (#25), what if it tries to catch ArgumentError if there is no relative_path_from(directory) and returns the original path if so?