PATCH: Add uri percent-encoding for binary data

Hello

Attached is a patch for adding uri as an encoding option for
encode/decode. It uses what's called "percent-encoding" in rfc3986
(https://tools.ietf.org/html/rfc3986#section-2.1)

The background for this patch is that I could easily build urls in
plpgsql, but doing the actual encoding of the url parts is painfully
slow. The list of available encodings for encode/decode looks quite
arbitrary to me, so I can't see any reason this one couldn't be in
there.

In modern web scenarios one would probably most likely want to encode
the utf8 representation of a text string for inclusion in a url, in
which case correct invocation would be ENCODE(CONVERT_TO('some text in
database encoding goes here', 'UTF8'), 'uri'), but uri
percent-encoding can of course also be used for other text encodings
and arbitrary binary data.

Regards,
Anders