fix: various type declarations
fix: various type declarations

* `clone()` and `with()` now return `this` type
* type sig added for `getMergedOptions()`
* made `endpointName` optional in FetchOpts
* make Opts more open to allow for custom opts
* run typedefs thru prettier --single-quote

Fixes: https://github.com/groupon/gofer/issues/104
Fixes: https://github.com/groupon/gofer/issues/105

groupon/gofer