add prefer-is-empty rule

[AndreaPontrandolfo]

Added prefer-is-empty rule. It enforces the use of isEmpty Lodash method over the native syntax. The rule is accompanied by tests.

[coveralls]

Pull Request Test Coverage Report for Build 724

• 34 of 41 (82.93%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-1.0%) to 99.022%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/rules/prefer-is-empty.js	34	41	82.93%
<!--	Total:	34	41

Totals
Change from base Build 723:	-1.0%
Covered Lines:	1033
Relevant Lines:	1040

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💛 - Coveralls