Implement sys_path_prepend option

Implement sys_path_prepend option

Added new config file option ``prepend_sys_path``, which is a series of
paths that will be prepended to sys.path; the default value in newly
generated alembic.ini files is ".".  This fixes a long-standing issue
where for some reason running the alembic command line would not place the
local "." path in sys.path, meaning an application locally present in "."
and importable through normal channels, e.g. python interpreter, pytest,
etc. would not be located by Alembic, even though the ``env.py`` file is
loaded relative to the current path when ``alembic.ini`` contains a
relative path.

Change-Id: If8f4279bd862acca44d46f4b1ab90b0a18098af3
Fixes: #797