Only accept options object in mutate

[FernandoChu]

Currently mutate allows for the option object to just be a boolean, representing the value of revalidate. While this is useful since most of the time that's the only option one cares about, it is also very confusing for function to take two different types as options value. This behavior is not documented in the docs, I'm not sure if this is because this is not officially supported, in which case it would be good to remove this for v2.

Related: vercel/swr-site#328