[FEATURE] Create access for isopleth artist kwargs

[ESadek-MO]

Currently (once subplots is in), to access line, you have to go via the kwargs dictionary. There are multiple others, and this is pretty inconvenient.

Could classes be a solution?

[ESadek-MO]

I'm moving this to 0.5.x.

It's slightly more complicated than I thought. The superclasses already have self.text() and potentially self.line. This makes it a complicated name space.

I could just create the variables self.line_config, but that doesn't strike me as much better than self._kwargs["line"], as it's equally unaccessible.

I don't want to hold up subplots any longer.

EDIT: I've gone back one this. There's a PR up now (#244). It's the middle ground I spoke of above. I'll leave it up to the reviewer if it's worth it.