Added plugin to read Windows performance counters

[TheFlyingCorpse]

Plugin that reads performance counters from Windows, a lot of examples included to get an idea how to use this with IIS and AD to start with.
The plugin is flexible so you can have a master configuration to push out to all, it will only submit from each host what is matching to your configuration.

[sparrc]

remove commented import

[sparrc]

Thanks a bunch @TheFlyingCorpse, this is great for windows support 👍!

A few things to address:

1. Write a README please!
2. Can you unit test some or all of this?
3. Put a dummy file called win_perfcounters_notwindows.go with the // +build !windows flag in the directory, otherwise unix builds will complain that there are no buildable source files in the directory.
4. Are all of those examples necessary for the config file? Could you maybe cut those down by at least half and then put the rest in a README file?
5. Try to reduce line-length to 80 characters throughout
6. If you happen to have any useful "recipes" of combinations of perf counters that you commonly use, that would be great to have in the README as well. Such as "cpu monitoring" and then give an example of the objects users might want to monitor the cpu.

[TheFlyingCorpse]

I'm still working on the unit tests, will try to have something tomorrow!

[sparrc]

@TheFlyingCorpse Is this ready to merge?

[TheFlyingCorpse]

@sparrc - Yes, I have formatted the code to the best of my ability and added unit tests for both the configuration parsing and doing test queries on Windows.

[sparrc]

@TheFlyingCorpse can you please squash your commits and rebase off master?

[TheFlyingCorpse]

@sparrc, done!

[sparrc]

thanks @TheFlyingCorpse! this is in master now

[sparrc]

note: I changed the name to win_perf_counters for consistency