Adds meta tags to support firefox plugin

I'm working on a firefox plugin to support restartable downloads via onionshare (see https://github.com/joshuathayer/onionshare-plugin). This diff adds meta tags to the onionshare download page to enable the plugin to more easily discover downloadable files.

This diff also adds an optional command-line flag (--local-only) which avoids setting up the hidden service, instead serving the file only on a local port, which makes development of things like the plugin easier.