http_header_envs option to include http headers as an env var (following CGI standards)

[mariobuikhuizen]

Continuation of #835 with CGI standards applied.

To try it out:

voila --VoilaConfiguration.http_header_envs=uSeR-agEnt my_notebook.ipynb

in my_notebook.ipynb

import os
print(os.environ.get('HTTP_USER_AGENT', 'User agent env var not found'))

[jtpio]

Thanks @mariobuikhuizen and @danlester!

[jtpio]

Kicking CI

[maartenbreddels]

Thanks @jtpio !