This coherent with what is done for Table. All platform specific things
are grouped into an options array. Eventually flags should be migrated
into options as well.
Support for Partial Indexes was available in Doctrine 1 following
http://www.doctrine-project.org/jira/browse/DC-82. This commit
reintroduce support for Doctrine 2. We use the same syntax with an
optionnal "where" attribute for Index and UniqueConstraint.
