Add support for standard flag

[karthikraobr]

This PR adds support for determining if a package is a part of the standard library.

[uudashr]

Is there any use case for this?

[karthikraobr]

Is there any use case for this?

microsoft/vscode-go#2803 especially this comment I am not sure if this could be considered a good enough use case?

[uudashr]

Ramya suggests to use Dir and compare with GOROOT, the workaround should works. Do you think we still need to expose {{.Standard}} or exposing it is the better way?

[karthikraobr]

Ramya suggests to use Dir and compare with GOROOT, the workaround should works. Do you think we still need to expose {{.Standard}} or exposing it is the better way?

I am not quite sure. I already am working on the Dir workaround. Do you think adding {{.Standard}} would add a significant overhead? If not I guess we could keep it right?

[uudashr]

Should ok. We still have fail on travis check here. I might not available in around 2 or 3 days, so it would be good if you can check and fix it so I can merge your changes as soon as possible.

[nezorflame]

Travis will be fixed once #18 is merged