Add EnvironmentContext and Config to MigrationContext

This lets migrations do stuff like:

    op.get_context().config.get_main_option('schema')

where `schema` is a custom option that I added to alembic.ini
Merge branch 'add_env_context_and_config_to_migration_context' of https://github.com/msabramo/alembic into t
- changelog + test for MigrationContext.config