Fix test command when using docker

[BBlackwo]

• backstop test --docker was actually running reference instead. Fixed that and did a bit of refactoring so there's less copied and pasted code.
• If you have spaces in a command it now adds quotes around it when passing through to Docker. E.g. backstop test --docker --filter="some test".
• Now --docker will work on window as I'm using ${process.cwd()} instead of $(pwd) in the docker command.
• Also added a CONTRIBUTING.md guide. It's a start. I couldn't figure out what commands to run and such to test my changes. Had to dig through the package.json > scripts. I think it could help others too.

[BBlackwo]

Any chance you can merge this one in @garris? It's been sitting around for almost a week now.

[brendonbribeiro]

@BBlackwo I'm having some trouble running with docker.

if (config.args.docker) {
    const passAlongArgs = process.argv
                  .slice(3)
                  .join('" "') // in case of spaces in a command
                  .replace(/--docker/, '--moby');

And in my case, i don't want to use --docker, i want set this on my configuration file.
I know that this have nothing to do with your code, but maybe you could give the user the choice of running with --docker or set it on configuration file.

What do you think?

[garris]

Thank you for the PR — this is a nice refactor. My only comment is around the contributing doc — there are some notes in the readme for contributing— maybe we can just tell the user to go here...
https://github.com/garris/BackstopJS/blob/master/README.md#developing-bug-fixing-contributing

I am planning to release the next version on the 24th. This change will be in for that one.

Cheers.

[garris]

@BBlackwo Just occurred to me -- you're adding quotes to docker args in this PR... What happens if the user included these quotes in the original argument? 👉 e.g. --filter="myScenarioName"

https://github.com/garris/BackstopJS/pull/831/files#diff-0745f5ef38ef41d5c870fa5515ddfd83R7

[BBlackwo]

@brendonbarreto being able to set it on the config file sounds reasonable. Maybe you can submit a PR for that :)

@garris thanks for the merge. Sorry I didn't see that section in the readme.

Also with the quotes, before my change if you ran --filter="My Scenario Name" the quotes would be stripped off when running the docker command. My change always puts them in case of spaces. The process.argv array didn't include the quotes in the arg, but just treated each arg as a string.

[BBlackwo]

Added it also to the table of contents in #838