Support metrics with count

[bartveneman]

css-analyzer has support for metrics with count, so we could choose to support that.

Use cases

• It will help projects transitioning if a color like #aaaaaa may still appear in some old module, but should not appear more often than X times, if we know that the legacy module contains this color exactly X times.
• As a developer I know that I use the error color #ff0000 exactly once, so I expect it to appear exactly once in my final output. Pre- or post-processing should not accidentally introduce this color more often
• We know that -ms-text-size-adjust should only be used once in our project, so we'd like to be able to lint that.

Caveats

• The 'old' config file should be supported, so we end up with having to check for both ['#aaa', '#bbb'] and [{count: 3, value: '#aaa'}, {count: -1, value: '#bbb'}], with -1 meaning that the color may appear infinitely.