(chore): Update generate:component script to use plop

Update yarn generate:component to use plop to generate component boilerplate.

Component name should be snake_case
Bootstrap Component name should be e.g. BTable

Template contents still need some work, but nicer to do than manually creating these files.

Example file output:

simon@delaware gitlab-ui % yarn generate:component
yarn run v1.22.0
$ plop
? [PLOP] Please choose a generator. Create Component - Create basic empty component
? Component name in snake_case, e.g. progress_bar:
 some_new_component
✔  ++ /src/components/base/some_new_component/examples/some_new_component.basic.example.vue
✔  ++ /src/components/base/some_new_component/examples/index.js
✔  _+ /index.js
✔  _+ /documentation/components_documentation.js
✔  _+ /src/scss/components.scss
✔  ++ /src/components/base/some_new_component/some_new_component.stories.js
✔  ++ /src/components/base/some_new_component/some_new_component.scss
✔  ++ /src/components/base/some_new_component/some_new_component.vue
✔  ++ /src/components/base/some_new_component/some_new_component.md
✔  ++ /src/components/base/some_new_component/some_new_component.documentation.js
✨  Done in 6.32s.