Add support for using TLS with PostgreSQL (#260)

[mossbanay]

Hi, I would like to use Refinery with a PostgreSQL database over TLS similar to (#260).

It looked like this wasn't too difficult to implement, and seems to work fine from my local testing. I've added a use_tls option that can be supplied in the toml config, and it also supports the original request of parsing from the connection string (though only disable and require are supported).

I don't often write Rust, let me know if you have any suggestions about this change.

Thanks

[mossbanay]

I think some of the CI plans checking fmt fail because openssl packages aren't installed in the base image. I was a bit worried this might happen, and same might be true of other people already using the package elsewhere. Might be preferable to explicitly enabled postgres tls as a feature then? What do you think?

[aig787]

@mossbanay any chance you'll have time to finish this up? If not I'm happy to take a swing at it.

[mossbanay]

@aig787 Sorry slipped through my inbox. By all means go for it!