Consider adding a "URL" input command-line argument to Smoketest

[mcbridem]

It might be neat to have a command-line argument that can accept a URL to test. This would make it somewhat easier to run quick tests. Currently you can create a text file that contains only URL and then pass that. For example:

test.txt contents:

https://www.usnews.com/education/arab-region-universities/king-saud-university-503764

Then you can test this with:

$ smoketest -vv test.txt

Instead, we could have something like this:

$ smoketest -vv --url="https://www.usnews.com/education/arab-region-universities/king-saud-university-503764"

However, this might be a bit messy for a few reasons.

One, we potentially want to support "remote expectations" via URL, so that you could do something like this:

$ smoketest -vv --include-url="https://code.usnews.com/its/smoketest/blob/master/expectations/anthill-out.yaml"

We want to make sure that any command-line arguments we add don't limit future features.

Two, we currently expect expectations input files to be a required and implicit command-line argument, so we'd need to make sure we test and account for any behavior changes.

Three, any command-line URL argument would likely be limited to HTTP 200 responses and could not do additional tests, such as checking HTML or HTTP headers.