WIP BibIndex: refactoring and author parent tokenizer

* Refactoring of BibIndex: implements https://github.com/inveniosoftware/invenio/issues/1710

* Moves many of the index-specific special cases to the tokenizers

* Adds tokenizers BibIndexAbstractParentTokenizer, BibIndexCanonicalAuthorTokenizer, BibIndexExactParentAuthorTokenizer, BibIndexParentAuthorTokenizer, BibIndexParentCanonicalAuthorTokenizer

* Implements https://github.com/inveniosoftware/invenio/issues/1885

* Adds indexes `simpleauthor`, `canonicalauthor`, `parentauthor`, `parentcanonicalauthor`, `exactsimpleauthor`, `exactparentauthor`

* Transforms `author` and `exactauthor` indexes from regular to virtual index

* Adds tests for tokenizers, for bibindex CLI, for bibindex utils

* Solves https://github.com/inveniosoftware/invenio/issues/1662


:warning: Still **WIP**, here for testing and review