Preserve generic types for ReadabilityOptions.serializer

[laymonage]

The changes in #971 seem to have lost the generic type parameter for the serializer option, enforcing the function's return type to be string instead of allowing it to be generic.

Per the README:

• serializer (function, default el => el.innerHTML) controls how the content property returned by the parse() method is produced from the root DOM element. It may be useful to specify the serializer as the identity function (el => el) to obtain a DOM element instead of a string for content if you plan to process it further.

[gijsk]

Thanks, this makes sense to me.