[spec] gemini capabilities

It has been made abundantly clear that Gemini is meant to be a frozen, 
immutable protocol ? without any possible extensions whatsoever. 

The forever protocol.

So be it.

Nevertheless, here is the outline of a small extension mechanism for 
Gemini: CAPA, short for capabilities.

CAPA manifest itself as a directive, expressed as an URN:

urn:gemini:capa

The CAPA directive itself returns a list of capabilities supported by a 
given service, if any, or an error code otherwise:

C: urn:gemini:capa
S: 20 text/gemini
=> urn:gemini:capa
=> urn:gemini:capa:connection:keep-alive
=> urn:gemini:capa:transfer-encoding:chunked
=> urn:gemini:capa:turn
<end of connection>

In this case, the service understand CAPA itself, plus a handful of other 
capabilities, such as persistent connection, and chunked transfer encoding.

A client enable a give capability by sending the relevant directive to the service:

C: urn:gemini:capa:connection:keep-alive
S: 10 Continue

To which the conformant service replies with an informational status code 
?10 Continue? acknowledging the new capability.

In this example ?with the keep-alive capability enabled? the network 
connection is persistent and not dropped to indicate end-of-transmission 
after each request.

Additionally, keep-alive enables the chunked transfer encoding ?  as now 
the Gemini service needs a different mechanism to indicate 
end-of-transmission to the client.

Wrapping all this together:

C: urn:gemini:capa:connection:keep-alive
S: 10 Continue
C: gemini://host.xyz:/life-is-a-long-quiet-river.gmi
S: 20 text/gemini;charset=utf-8;connection:keep-alive;transfer-encoding=chunked;
4\r\n
Wiki\r\n (data)

6\r\n
pedia \r\n (data)

E\r\n
in \r\n
\r\n
chunks.\r\n 

0\r\n 
\r\n 
<service maintain connection, awaiting next request>

That's all folks.

?0?