Net::Support parameters for MAIL FROM and RCPT TO

[c960657]

In Net::SMTP, add support for parameters for MAIL FROM and RCPT TO, such as SMTPUTF8 and REQUIRETLS.

I suggest extending Net::SMTP#mailfrom and Net::SMTP#rcpto so they accept an additional optional Array or Hash of parameters.

For Net::SMTP#send_message and Net::SMTP#open_message_stream, I suggest that in addition to a String email address (or arrays of Strings), these methods should accept a pair (or arrays of pairs) of [addr, params], where addr is the String email address, and params is an Array or Hash of parameters.

In order for the parameters to be useful, we should expose the capabilities reported by EHLO, so capable? should be made public.

[c960657]

New PR here: ruby/net-smtp#18