Add std YAML decoding and yaml/yx transform command

[orlade-anz]

Fixes #507.

Changes proposed in this pull request:

• Add //encoding.yaml.decode to std library. Virtually identical to //encoding.json.decode.
• Add yaml/yx command to transform YAML. Virtually identical to json/jx command.

Checklist:

• [x] Added related tests
• [x] Made corresponding changes to the documentation

[coveralls]

Pull Request Test Coverage Report for Build fec98dceeaa0eef05dcc731485b4bac9defbe2a4-PR-508

• 18 of 18 (100.0%) changed or added relevant lines in 2 files are covered.
• 5 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.1%) to 47.888%

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Files with Coverage Reduction	New Missed Lines	%
rel/value_set_str.go	5	62.57%
<!--	Total:	5		-->

Totals
Change from base Build 3d6dc0fd20c4e8265cbf894dee0eed319e1cd56a:	0.1%
Covered Lines:	4399
Relevant Lines:	9186

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💛 - Coveralls