Add `main` property to package.json

Allows requiring the module by its ID and not by `./node_modules/bootlint/src/bootlint`.

See https://www.npmjs.org/doc/files/package.json.html#main and also https://github.com/zacechola/grunt-bootlint/blob/ff2d7efc506c0e8ba2b2f4df907d617d8ff36ccd/tasks/bootlint.js#L12.