Add --splice support to merge command

Add --splice support to merge command

Added ``--splice`` support to the :func:`.merge` command. Previously, the
merge command would suggest using ``--splice`` when attempting to merge
non-head revisions, but the flag was not actually accepted by the command.
The ``splice`` parameter is now available in both the command-line
interface and the :func:`.command.merge` function, matching the existing
support in :func:`.command.revision`.  Pull request courtesy Kadir Can
Ozden.

Fixes: #1712
Closes: #1794
Pull-request: https://github.com/sqlalchemy/alembic/pull/1794
Pull-request-sha: a0f9b6d929b7124fbc9c4c0d4d8191f57a42222d

Change-Id: I12125061777b829b98663adab1701154c6f7c3ac