Add cli options to pass cert information

[chiragpat]

Why

Currently the only way to pass custom cert information to autocannon is via the tlsOptions in the autocanno js api. This PR adds the ability to pass in common cert information from the cli tool so that consumers can easily use autocannon without writing a custom script.

How

• Add logic in autocannon.js to parse arguments and read the relevant files
• Normalize all the arguments to be part of the tlsOptions key so that the httpClient.js can get the correct values

[mcollina]

lgtm