Make it possible to read file from a online source

[maxmannen]

Would be nice to be able to use a css file that resides online (Like a shared Google Spreadsheet).

For example

 csv do
  file_url 'some_url'
  separator '-'
  columns [:id, :name]
end

[zachdaniel]

seems like a neat idea, just a matter of implementing it. In all honesty I probably won't have the time/inclination to do this, but PRs welcome!