Add tags support

[pedrofaria]

with tags support you will be able to execute goverage passing tags parameters to go test.

eg.: goverage -v -race -coverprofile=cover.out -covermode=atomic -tags=integration

[pedrofaria]

This should fix #9 issue

[haya14busa]

go test itself supports testing multiple packages nowadays. Maybe you should use go test.

I don't remember the detail, but it's not actually so simple to support tag as in #12