Add memory.ItemizedPercent

[yudai]

Hi, thank you for the handy library.

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This new method returns a new struct PercentStat that holds CPU usage
percentages for different work loads (e.g. user, system and so on).
This commit also exposes CalculatePercent() and CaluculateItemizedPercent()
that return usage percentages from two TimesStats so that users
can calculate percentages themselves as well.

[shirou]

Thank you for your PR. But I think we should keep this library as simple as possible. And it seems this PR provide utility function, so how about to create another library which uses gopsutil?

[yudai]

Thank you for the reply. I understand keeping a library simple is always the way to go.

However, I'm afraid I assume this is a common use case. Actually I just found psutil has the same functionality.

https://psutil.readthedocs.io/en/latest/#psutil.cpu_times_percent
giampaolo/psutil#362

What if we simply rename ItemizedPercent() added in this PR to CPUTimesPercent() and make it return TimesStat instead of new PercentStat (IMO, it's better/safer to separate TimeStat and PercentStat as they store different types of values, but it's not critical to me)?

CalculatePercent() and CaluculateItemizedPercent() are just to expose existing logic in cpu.go for uses. We can drop these from the PR if you'd like.

If the idea above works for you, I'll update my code. (EDIT: updated)