Send login_options and dynamic dots to curl::send_mail

[guslipkin]

As it stands, {blastula} defaults to the most secure authentication method available on a given smtp server. However, this may not always be desirable since the more secure methods may not be properly provisioned even though they are available. Allowing users to pass the login_options argument to the underlying curl::send_mail function would resolve this authentication issue, as well as allow users to specify any other login options according to the curl manual.

In addition, allowing users to supply dynamic dots to be passed to the curl::send_mail function would allow users to make any modifications they need through the friendly interface that {blastula} provides without needing to fall back to curl::send_mail themselves.

A pull request to allow dynamic dots was previously submitted in #249

There will be a pull request to accompany this issue.