Serve text-type assets from sketch asset urls - Fixes #839

[meiamsome]

I have verified that this pull request:

• has no linting errors (npm run lint)
• is from a uniquely-named feature branch and has been rebased on top of the latest master. (If I was asked to make more changes, I have made sure to rebase onto master then too)
• is descriptively named and links to an issue number, i.e. Fixes #123

Potentially will also resolve some other issues (See the discussion in #839) I haven't verified those use cases work though, so will refrain from tagging them with the 'fixes' keyword

[catarak]

cool! this is working for me. i tested this with files with a url and those are working as well. going to merge this!