Add 'openssl req' option to specify extension values on command line

The idea is to be able to add extension value lines directly on the
command line instead of through the config file, for example:

    openssl req -new -extension 'subjectAltName = DNS:dom.ain, DNS:oth.er' \
                     -extension 'certificatePolicies = 1.2.3.4'

Fixes #3311

Thank you Jacob Hoffman-Andrews for the inspiration

Reviewed-by: Andy Polyakov <appro@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4986)