Option to exclude godeps directories

[f1yegor]

I can see some godeps files and other unrelated stuff like

• Godeps/_workspace/src/github.com/prometheus/client_golang/prometheus
• github.com/prometheus/prometheus_cli/.build/gopath/src/github.com/prometheus/common/expfmt
• .build/gopath/src/github.com/Sirupsen/logrus

[KyleBanks]

Definitely, there's a slice of ignored paths here we could add those too (Godeps/ and .build/) since both would be something everyone would likely ignore.

In the future maybe a more customizable ignore system would be helpful but for now I think this is okay.

[f1yegor]

Looks good!