Add test utils

[pago]

Reference issue: #83

What changed in this PR:

Added our test-utils package and the Jest snapshot serializer.

[pago]

@CarsonMcKinstry ready for your review.

Note that we automatically do a release for each PR/commit so if you check https://travis-ci.org/trivago/melody/jobs/478175386 you'll likely be able to find an actual release that you can npm install if you want to test it a bit more. (does not work for new packages, had forgotten about that

[coveralls]

Pull Request Test Coverage Report for Build 488

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.02%) to 89.488%

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Files with Coverage Reduction	New Missed Lines	%
packages/melody-idom/src/core.ts	1	91.79%

Totals
Change from base Build 211:	-0.02%
Covered Lines:	2156
Relevant Lines:	2287

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💛 - Coveralls