Add CLI flag for `errors`

[connernilsen]

Add a CLI flag and environment variable overriding errors. See #458 for more details.

This one will likely require two new flags: an --enable-error and --disable-error flag (or whatever you think it should be named)!

[Alex-Aron]

I could take a look at this one!

[connernilsen]

Sounds good! Let us know if you run into any issues

[Alex-Aron]

Are two flags required? Would you rather me do --enable-error and --disable-error or --enable-error true/false?

[connernilsen]

Right now I'm leaning toward --enable-error and --disable-error, since --enable-error <error> true/false seems more verbose and less common from other type checkers/linters. I can be convinced alternatively if you have an argument for it though.

I'm also not too set on --enable-error and --disable-error as names, so if you have other ideas, please let us know! (Note: we may add warnings at some point in the future).

Maybe to start, we should also make it an error if something's in both enable and disable?