optional STARTTLS support

[akostadinov]

fixes #142

You can test by:

openssl req -new -newkey rsa:4096 -days 365 -nodes -x509 -subj "/C=US/ST=Denial/L=Springfield/O=Dis/CN=localhost" -keyout certs/localhost.key -out certs/localhost.cert

bundle exec mailcatcher --smtp-starttls --smtp-private-key certs/localhost.key --smtp-cert certs/localhost.cert

openssl s_client -starttls smtp -connect localhost:1025

AUTH PLAIN XDB0ZXN0X3VzZXJAd29zaHViLmNvbVwwKW5n

Auth is just a bonus tip actually. By default eventmachine smtpserver accepts any username/password pair in PLAIN auth.

[onpaws]

@akostadinov Thanks for your efforts here!

Is there a Docker image anywhere I could try?
I'm curious to try this out but don't have Ruby on this machine at the moment.

[akostadinov]

I don't have. You need to either install ruby or enter a shell of a suitable existing container image.

[daniloargentiero]

Please merge this,
thank you.

[robbl-as]

@akostadinov I think this PR has a higher chance to be merged, when documentation is added. Do you mind adding the new options to the README.md?

[akostadinov]

@robbl-as , I don't really have time and I'm not very good at writing docs so it costs me a lot of stress. If you like, you can submit a PR against my branch and I will merge it. Or paste a patch as a comment.