Make meek-server easy to use with Let's Encrypt
Currently it's not trivial to get certificates for meek-server using Let's Encrypt. The
--webroot option, for example, wants to write a token to the filesystem so the web server can serve it, but meek-server doesn't serve files from the filesystem.
Ideally this works in a way that certificates can be renewed (e.g. in a cron job) without restarting tor or meek-server.