Support -E, --cert and --key options

[kanjibates]

Add support for SSL/TLS client certificates and private key.

Client certificate (-E, --cert)

• https://curl.se/docs/manpage.html#-E
• https://docs.guzzlephp.org/en/stable/request-options.html#cert

Private key (--key)

• https://curl.se/docs/manpage.html#--key
• https://docs.guzzlephp.org/en/stable/request-options.html#ssl-key

Fixes #28, #32.

[jasonmccreary]

Thanks. Sorry for the delay. I was not receiving notifications for this repo.