feat(config): introduce deprecation warning handler, fix #1672

[xanf]

Allow passing custom handler for deprecation warnings allowing fine-grained control how these are reported to a user.
This is super important for huge codebases, where simple console.error reporting is not enough. For example, for gradual migration I would like to turn deprecation reports one-by-one, while our codebase is cleaned, making sure they will never appear. Currently this is possible only by hooking into console.error, which is bad, "stringly-typed" and somewhat hard to implement

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

• Bugfix
• Feature
• Code style update
• Refactor
• Build-related changes
• Other, please describe:

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

• Yes
• 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

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature (to avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it)

[lmiller1990]

Cool. I will release this next Monday. Please wait a bit.