Fix type defs for .with() and .clone()

[dbushong]

The return values for

gofer/lib/typedefs.d.ts

Lines 23 to 25 in 02c190f

	clone(): Gofer;
	with(opts: Gofer.Opts): Gofer;

shouldn't be Gofer - they should be whatever the subclass is - I'm not sure how to properly declare this in typescript

[jkrems]

There's the magical this annotation that may help.

• Playground Link
• Docs on polymorphic this types

[dbushong]

Thanks! I saw the this syntax, but assumed it meant you had to be returning the exact same object - but I guess it really doesn't matter from a typing perspective - at least at the level of detail we're currently using.