Add debug function

[SergiCL]

This PR contains the changes discussed in #1461

What kind of change does this PR introduce? (check at least one)

• Feature

Does this PR introduce a breaking change? (check one)

• No

The PR fulfills these requirements:

• It's submitted to the dev branch.
• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)
• All tests are passing: https://github.com/vuejs/vue-test-utils/blob/dev/.github/CONTRIBUTING.md#development-setup
• New/updated tests are included

[lmiller1990]

Great work. I personally do not want to make the API even bigger at this point - I also think debug is the wrong name for this function, since it does not debug anything, it just prints html. debug suggests it will drop you into a kind of console debugger (like pry from perl/python/rub or a JS debugger, like you get in IDEs such as WebStorm.

For this reason, I think this makes way more sense as an option to html. Eg: console.log(wrapper.html({ pretty: true }). We could also set this via a config variable to make this the default, is the users chooses to do so.

@afontcu @JessicaSachs @dobromir-hristov thoughts?

[dobromir-hristov]

Yeah same.

[Stoom]

It looks like the current wrapper.html() already pretty prints. From the look at the code, this just does a console.log (which doesn't seem to be desirable) and loops through a WrapperArray.

[SergiCL]

I'm going close this to avoid confusion and open a new one with the changes described in #1461 when done.