Feature: Specify output directory in CLI

[schneems]

After rundoc is finished running, it copies the contents to a folder. It would be nice to be able to specify which one in the command line like rundoc build --path <rundoc.md> --output-dir ./lol.

I think we also want to be safe about it, i.e. check that the contents of that directory won't be over-written before writing there so if someone accidentally runs --output-dir /usr/local or something it won't bork the whole computer. It should check if the directory is empty first and error if it's not. It should create it if it does not exist.

Likewise we should also have a --debug-dir configuration (naming?, this is effectively what tmp is now), but default it to the same root location as the output dir but with tmp/.