Config-file

Added

- More type hints to `ConfigFile` and `IniParser`.
- `nested_lookup` dependency to help with modifying deeply nested structures
(JSON + YAML)
- `JsonParser` so you can now specify `.json` files to be parsed.

Changed

- The original content of the passed in file is now called `content`
instead of `contents`. This is for consistency since the parsed version is
called `parsed_content`.

Fixed

- Support for using custom parsers with `ConfigFile` with the `parser` optional
argument. This was technically supported before, but it was not tested and found
to not actually use the passed in parser once tested.

Changed

- The `reset()` method on `ConfigFile` is now called `restore_original()`.
The behavior is the same. This was done to better describe what exactly that method
is doing. Since the file is not written back out with every `set()` or `delete()`
and calling `save()` explicitly is required, `reset()` may have been confused with
resetting the changes you've made rather than deleting and restoring the original
configuration file.

Changed

- Bumped down python version requirement to 3.6 and now test 3.6, 3.7, and 3.8 on CI.

Added

- Support for retrieving entire sections as a `dict` with `get()`.

Added

- `reset()` and `save()` methods to `ConfigFile`. This allows you to reset your
configuration file to an "original state," given the original config file path.
However, say you have a `config/config.json` file. Then it will automatically try
to look for `config/config.original.json` if no file path is specified. The `save()`
method should be called after your changes to the config file. It will write them
back out.

- Raise test coverage to 93%.

Changed

- `_split_on_dot` is no longer in base parser (now in a utils file). Also, the default
behavior is now to split on every dot and split on only the last dot if specified

Fixed

- `ConfigFile` was trying to use `_split_on_dot`, but it no longer inherited from base parser.

- `TypeError` when setting the key/value pairs since configparser requires the option
to be a string. The parser now just converts the value to a string if it is not one
and then adds the key/value pair. It would still be parsed correctly when retrieving
it.