Publish alpha versions with `prerelease` tag

Currently, alpha versions are published with `latest` tag. This has the
following consequences:
- when installing the package, with `npm install handlebars` users
will receive alpha version which is most likely not what they want.
- `npm outdated` will report latest stable as outdated, but its not
really true - there is no new *stable* version to update to.

`publishConfig` must be removed, when releasing stable `2.0.0`