Fix return type when throwOnTimeout: true passed to constructor

[Richienb]

Fixes #214
Closes #218

[Richienb]

// @sindresorhus

[sindresorhus]

I think {throwOnTimeout: false} was a mistake. It's surprising that it's just returns undefined, and having it as an option complicates types. What do you think about just making it the default behavior in the next major and dropping the option?

[Richienb]

@sindresorhus throwOnTimeout: true makes sense. But whether to remove it depends on what the use cases of throwOnTimeout are. My use case doesn't rely on timeouts so idk.

[sindresorhus]

e48716f