percent_encoding::NON_ALPHANUMERIC should be configurable

[flo-at]

There are use cases where it does more harm (by affecting the readability of a URL) than good if too many non-ASCII chars are %-encoded. It would be possible to change that without breaking the API by adding Cargo features for the non-ASCII chars (or groups, as in the URL Standard). This would also enable projects to configure this set, that indirectly use this crate though a depedency.

If that's an option, I'll be happy to make a PR for this.