Add minimal MySQL version to Scraper interface

[AlekSi]

@arvenil's PR #265 introduced Scraper interface. Now we propose to expand it to include the following method:

type Scraper interface {
	// Version of MySQL from which scraper is available.
	Version() float64
}

It then can be used to skip unsupported versions entirely:

	for _, scraper := range e.scrapers {
		if versionNum < scraper.Version() {
			continue
		}

I can work on porting this feature from Percona's fork if you are interested.

[SuperQ]

Oh, that sounds great.