Support for $value and $type

[adamghill]

• Initial support for $type and $value operators mentioned in Database, Cache, and Email URLs in the style of 'environs' #2
• Refactored a bunch of code be a little more OO-ish
• Support for bool, int, str, float, decimal, datetime, date, time, timedelta for $type

[adamghill]

@LucidDan curious what your thoughts are for these "types" here: https://github.com/adamghill/dj-toml-settings/pull/8/files#diff-6c843a10de40f74a1a40e896fb85fa99c940d122390c19afda9603189733b44cR145-R185.

I handled the ones from environs that made sense to me personally. Questions:

• Getting back a ParseResult for "url" seems a little unintuitive
• Not sure what "json" would return (or how useful it would be)

Any other thoughts are welcome!

[LucidDan]

Work stuff has me unavailable for OSS stuff until the weekend, but I'll take a look on my Saturday!

[adamghill]

I think this is ready to review. I need to look into the mypy errors and get test coverage back to 100%, but I'm pretty happy with this in general.