Add support for CAPA command

[nevans]

This adds support for asking a POP server about its capabilities (RFC2449).

• Adds Net::POP3Command#capa, which issues the CAPA command and encodes the result as a hash of {tag => params for tag, *params in each_list_item}. Tags and params are both upcased, to simplify case insensitive matching.
• Adds Net::POP3#capabilities, which caches the result from command.capa.
• Adds Net::POP3#capable?, which returns whether #capabilities includes a specific tag and param.
• Adds Net::POP3#clear_cached_capabilities, which clears the capabilities cache.
• Adds Net::POP3#sasl_capable? and Net::POP3#sasl_mechanisms, which query support of the "SASL" capabability.

Note that the STLS extension modifies the CAPA requirements. If support is added for STLS, the capabilities cache must be reset.

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This is needed in order to properly support SASL authentication using the AUTH command (RFC5034).

And a basic SASL implementation should be the basis for supporting OAuth2 (whether via the non-standard XOAUTH2 SASL mechanism or the standard OAUTHBEARER SASL mechanism). Therefore, CAPA is a requirement for proper implementation of OAuth2.

[nevans]

I rebased this and changed it a little. Now it only add the #capa method to POP3Command, and the other methods are implemented directly on POP3.