Add RSS feeds (fix #57)

[sigaloid]

Fixes #57

Links don't work because we don't know exactly the full URL (can't do relative linking in RSS).

Todo:

• Config/compile gate
• Add config option for full domain URL (needs to be placed in relevant URLs)
• Properly add all RSS fields to better match official Reddit RSS feed, though some are flat out incompatible with (this) RSS library (rss - https://lib.rs/rss).

[nickelswitte]

Super nice you are working on this! thanks a lot!

[sigaloid]

Live from the Boston Rust hackathon...

Tada!
REDLIB_ENABLE_RSS=on REDLIB_FULL_URL=http://localhost:8080 cargo r (FULL_URL is necessary if you want the link to be accurate for self posts)