Support pretty diagnostic messages in CLI

[danaetroupe]

Closes #495: You can now use the --formatter prettier flag in the CLI to get prettier errors

NOTES:

• Characters may not show properly in certain outdated terminals
• Multi-line errors not supported
• See issue for more details about implementation

[JohnnyMorganz]

Sorry for the delay here.

This is looking pretty neat, and surprisingly simple implementation too, I guess everything we need is already there in the 3rd party repo :)

I'm actually curious, maybe we should enable this as the default output. I think it would be useful for most users.

Anything in particular you think this is still missing?

[JohnnyMorganz]

I wonder if the top line should include the error message, rather than just TypeError - i think that should go as part of the code.

[danaetroupe]

@JohnnyMorganz There are some terminal incompatibilities so I'd like to see what the results are on different machines. I want to look into other character sets. In my terminal, I had to manually allow it to show certain characters so I wonder if there is a way to detect that. Also, I think ultimately being able to group errors by file will be important. I can come back and take a look at those changes in depth but I think that should be a feature before enabling as default. If you are analyzing an entire project at once I could see this formatting becoming inefficient.