Add :condition option for .memoize method

[AlexWayfer]

Now you can specify a block for instance execute
whether to use memoization or not:

def file_mtime
  File.mtime asset_file
end

memoize :file_mtime, condition: -> { environment == 'production' }

It's useful for AlexWayfer/flame#22 (the branch is ready).

[coveralls]

Pull Request Test Coverage Report for Build 21

• 25 of 25 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 20:	0.0%
Covered Lines:	153
Relevant Lines:	153

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💛 - Coveralls

[tycooon]

Thanks for the PR!

[tycooon]

By the way, if you can add some docs to the Readme – that would be very nice.

[AlexWayfer]

By the way, if you can add some docs to the Readme – that would be very nice.

Yeah, sorry, I forgot about this. Done in #8.