module-aware building

Now Go 1.11 introduces module-get, I suggest to change `gopherjs build` or other commands to be module-aware. For example, I'd want the below code to be workable:

```sh
mkdir foo
cd foo
go mod init example.com/m
go get github.com/gopherjs/gopherjs
gopherjs build -tags=example github.com/hajimehoshi/ebiten/examples/sprites@master
```
