auto-register function

Each time we add a new component or directive, we need to manually import it in a bunch of places. Both for tests and stories here, and in gitlab-ui.js in CE repo.

To simplify this, we could use a register function. Same idea as https://github.com/bootstrap-vue/bootstrap-vue/blob/dev/src/utils/plugins.js#L7

If we did it in our main export then usage could be something like;

import gitlabUi from '@gitlab-org/gitlab-ui';

Vue.use(gitlabUi)

This way adding new components only requires changing the package version.