Optional steps

[tomakado]

• Steps in manifest now have new boolean field optional with default value false (manifest.Step.IsOptional);
• Use --include or -i flags to include optional steps. Example: -i manifest,dockerfile or -i manifest -i dockerfile;
• Use --all flag to include all optional steps (overrides --include).

[coveralls]

Pull Request Test Coverage Report for Build 1880790297

• 28 of 28 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+1.8%) to 89.691%

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Totals
Change from base Build 1873181836:	1.8%
Covered Lines:	261
Relevant Lines:	291

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💛 - Coveralls