Indexing or Unindexing single or multiple columns of a single table via migration generator

Sometimes happens that later in the development process one realizes that a few columns of a particular table should have been indexed.

This pull request easies the addition and removal of indexes of single columns on/from a single table by using the migration generator via CLI

The changelog as well as the code provides the cases of use.

Looking forward to your feedback.

Frank Pimenta