Add support for client and peer certificates

[gshuflin]

This commit adds two additional command-line flags for specifying SSL
certificates to be used by drill when making HTTP requests.

--cert is used to specify a client certificate file (following the
curl option with the same name). Again similarly to curl, --cacert
is used to specify a PEM format certificate that should be used to
verify the remote peer.

[4F2E4A2E]

This is a quite interesting PR, thank you, for that. For me, it is unclear why exactly the CI [1] is failing. Do you need a hand investigating that?

1: https://app.travis-ci.com/github/fcsonline/drill/jobs/506791339#L724

[corrieriluca]

The CI is failing because the cargo fmt --all -- --check fails, i. e. the formatting of the code is incorrect (here there are trailing new lines at the end of src/config.rs).

[gshuflin]

Looks like there have been some substantial changes to the existing code since the last time I was looking at it. This is a patch I implemented to solve a particular problem at a company I no longer work at and haven't thought about in a long time, so I'm not in a good position to debug the merge conflicts. If anyone else wants to take this code and create a working patch out of it, they are welcome to do so.