Add golint package name MixedCaps rule

[cce]

Is your feature request related to a problem? Please describe.
I have been using revive as a "drop-in replacement for golint" but recently noticed revive did not detect an issue I remembered golint used to raise. golint has a rule to check for MixedCaps package names, added in golang/lint#285 and implemented in https://github.com/golang/lint/blob/83fdc39ff7b56453e3793356bcff3070b9b96445/lint.go#L561-L563

Describe the solution you'd like
It would be great to have revive's var-naming rule also perform a check for MixedCaps package names.

Describe alternatives you've considered
I've considered re-enabling golint and running it alongside revive but that would result in duplicate linter suggestions.

Additional context
PR to follow.