add --project-dir flag to allow specifying project directory

In particular, this is the directory that contains the dbt_project.yml
file and all the related project files and directories.

Without this flag, it is necessary to cd to the project directory before
running dbt.

See #1544


automatic commit by git-black, original commits:
  2e7c1fd

core/dbt/main.py

@@ -239,7 +239,7 @@ def _build_base_subparser():
     base_subparser = argparse.ArgumentParser(add_help=False)
 
     base_subparser.add_argument(
-        '--project-dir',
+        "--project-dir",
         default=None,
         type=str,
         help='''