Add new rule: don't use `fmt.Sprintf` for assertion description

[nunnatsa]

As the gomega assertion methods (To, NotTo ToNot, Should and ShouldNot) are a formatting functions, that accepts a format string + parameters starting from the 2nd parameter, there is no need to use fmt.Sprintf for assertion description.

This PR add an optional rule to warn about using fmt.Sprintf as assertion description. It is not enabled by default. Use the -allow-sprintf=false flag to enable this rule.

Fixes #48

Not sure this is needed. See discussion in #48

/hold

[coveralls]

Pull Request Test Coverage Report for Build 4217053645

• 75 of 75 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.4%) to 88.563%

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Totals
Change from base Build 4215736834:	0.4%
Covered Lines:	573
Relevant Lines:	647

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💛 - Coveralls

[nunnatsa]

/hold

[nunnatsa]

/unhold

[nunnatsa]

/unhold

[nunnatsa]

/hold

[nunnatsa]

/hold

[nunnatsa]

Closing for now. I think it's overkill.