Support build tags in Magefile

[joeblubaugh]

What would you like to be added:
Add a new field to the build package's Config type called Tags that is a []string. The buildBackend step should apply these tags to the build using the -tags flag.

Why is this needed:
One use case is enabling different behavior based on a build configuration. I would like to change the definition of certain functions when I pass a -tag experimental to go build. The provided Mage setup has no convenient way to pass these tags.