Make useFetch() support reactive arguments

[lbssousa]

This is a suggestion based on a similar useFetch() implementation from project vue-async-function.

Instead of just invoking fetch() in component mounted lifecycle hook, you could make useFetch(url) support a reactive wrapper for url argument, so it could watch() for URL changes and trigger another fetch(url) call whenever url changes. This could be useful e.g. in search boxes, when url changes based on the text to be searched.