feat(pagination): `total-items` & optional `items-per-page` API

Closes #820.

BREAKING CHANGE: API has undergone some changes in order to be easier to use.
 * `current-page` is replaced from `page`.
 * Number of pages is not defined by `num-pages`, but from `total-items` &
  `items-per-page` instead. If `items-per-page` is missing, default is 10.
 * `num-pages` still exists but is just readonly.

  Before:

  <pagination num-pages="10" ...></pagination>

  After:

  <pagination total-items="100" ...></pagination>