Support for tagoptions

Namely, we're interested in JSON. I took the source code for parsing
struct tags from the Go stdlib, since the functions I'm interested in
are not publicly exposed[1].

Currently, it only supports the "omitempty" option and obviously a
tag's name.

[1] http://golang.org/src/pkg/encoding/json/tags.go?m=text
@kyrylo This should be a private function (tagContains).