v0.2.0

Adds ImportMap field to the GoNames type. This allows users to add custom mappings of proto file names to Go package options. There is also a new method, GoPackageForFileWithOverride, that allows the caller to supply the Go package option, instead of it being looked up from the file's go_package option or the newly added ImportMap.

This allows Go-generating plugins to easily support import_path and M... arguments, like protoc-gen-go does.