Add common time formats not found in the Go time system package

[stephen-soltesz]

This change adds a new package for extensions to Go's time package. In particular, we regularly want to use time formats not defined by the time package. Namely, YYYYMMDD, YYYY/MM/DD, and YYYY-MM-DD are common throughout M-Lab's data pipeline and other tools.

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This change is 

[coveralls]

Pull Request Test Coverage Report for Build 1167

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 94.264%

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Totals
Change from base Build 1161:	0.0%
Covered Lines:	2087
Relevant Lines:	2214

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💛 - Coveralls