v0.7.0 - allow setting different P4CHARSET for both source and destination servers

Thanks to @kalihman for pointing out the fact that p4harmonize wasn't handling a mismatch in CHARSET values between the client and server.

• Previous behavior: if p4's P4CHARSET was set, it was blindly used for both the source and destination servers.
• New behavior: there is a new p4charset field you can add to your config.toml files for both the source and destination servers, and that will be passed to p4 as a -C global option, overriding whatever P4CHARSET would otherwise default to. If you do not add those fields to your config files, the behavior should be unchanged.

Also note that apparently the VS Code Go extension has new linting suggestions that I decided to follow, and the only user-facing change is error messages now start with a lower-case letter instead of an upper-case letter. Apologies if this breaks anyone's workflow!