Improved extensibility of the URI class by modifying `_uri_assoc()` to (1) not force a specific list of valid options for `$which`, and (2) build the `$segment_array` and `$total_segments` dynamically regardless of the contents of `$which`.

Signed-off-by: Daniel Hunsaker <danhunsaker@gmail.com>
Merge pull request #2106 from danhunsaker/feature/uri-extensibility

Improved extensibility of the URI class