support paths in file_template

support paths in file_template

The ``file_template`` configuration option now supports directory paths,
allowing migration files to be organized into subdirectories. When using
directory separators in ``file_template`` (e.g.,
``%(year)d/%(month).2d/%(day).2d_%(rev)s_%(slug)s``), Alembic will
automatically create the necessary directory structure. The
``recursive_version_locations`` setting must be set to ``true`` when using
this feature in order for the revision files to be located for subsequent
commands.

Fixes: #1774
Change-Id: Id68a3b0483c6519d724bbb79bbf8b471ea697c36