Add custom GOPATH setting

When vendoring dependencies, the linter needs to use a different GOPATH for each project. I've added an optional 'gopath' setting, which can now be set in a `.sublime-project` file:
```json
{
	"SublimeLinter": {
		"linters": {
			"gotype": {
				"gopath": "/custom/go/path:/another/go/path"
			}
		}
	}
}
```
Let me know what you think, thanks for the good work.