Add support for TLS session resumption using session tickets

[sovrynn]

Background

Hello, vegeta is used by my team for HTTP(s) performance testing. Recently, I needed to test the performance improvements of TLS session resumption using session tickets, but wasn't able to find a good HTTP benchmarking tool that supported it. So, I thought that this would be a good feature to add to vegeta.

I have manually tested the feature, added a test for it, and updated the README.