Move parsing into parse subcommand (introduce argparse)

[meshy]

We intend to add more functionality to this tool, so it no longer makes sense to have the error parsing functionality at the top level when calling the script.

This change introduces (and requires) a new parse subcommand, under which the existing parsing function is moved. This change is backwards incompatible.

Because we're using argparse to do this, we also get a new "--help" flag on the command.

We also add a new --indentation flag to the parse subcommand which grants control over the indentation in the JSON output.

This is spun out of #18, which will introduce a new command.

[Tenzer]

This broadly looks good! I had a go at adding an argument for specifying the level of indentation used in the JSON output, just to test this out, and it was very easy:

diff --git a/mypy_json_report.py b/mypy_json_report.py
index f7db976..1163d28 100644
--- a/mypy_json_report.py
+++ b/mypy_json_report.py
@@ -42,6 +42,7 @@ def main() -> None:
     )

     parse_parser.set_defaults(func=_parse_command)
+    parse_parser.add_argument("-i", "--indentation", type=int, default=2, help="Number of spaces to indent JSON output")

     args = sys.argv[1:]
     parsed = parser.parse_args(args)
@@ -51,7 +52,7 @@ def main() -> None:
 def _parse_command(args: object) -> None:
     """Handle the `parse` command."""
     errors = parse_errors_report(sys.stdin)
-    error_json = json.dumps(errors, sort_keys=True, indent=2)
+    error_json = json.dumps(errors, sort_keys=True, indent=args.indentation)
     print(error_json)

[meshy]

This broadly looks good! I had a go at adding an argument for specifying the level of indentation used in the JSON output, just to test this out, and it was very easy:

Thanks! The recommendation to use argparse was a good one, for sure.

I really like the idea of this indentation flag -- it'll make things easier in environments with CI or pre-commit hooks that have opinions on indentation. I'll add that into this PR in a sec.

[Tenzer]

LGTM!

[meshy]

Thanks! I've got a couple more PRs to open still, so I'm not going to push a release tag yet, but I'll merge this now to make them easier.