Provider assets

[aleksrutins]

This pull request adds static assets support for providers. It uses a new static_assets method in Provider that returns a HashMap with filenames as keys and file contents as values. It puts files into /assets in the Docker container, and you can get the path to an asset by calling app.asset_path(name: &str).

See #158

[coffee-cup]

This is awesome! A few small comments.

Also, this doesn't seem to work with sub directories (i.e. specifying dir/hello.txt as the filename). It errors when attempting to copy the files. I think we should support this case or show a better error than

[aleksrutins]

Should all hopefully be resolved now.

[coffee-cup]

Thanks :D