Add --package flag to write __init__.py files

Add --package flag to write __init__.py files

Added new flag ``--package`` to ``alembic init``.  For environments where
the Alembic migration files and such are within the package tree and
importable as modules, this flag can be specified which will add the
additional ``__init__.py`` files in the version location and the
environment location.

Change-Id: I4993d17936f2b8a9833a28f0788389b19ff4333e
Fixes: #463