Improve parse_args (#2)

This solves two minor issues of the `powrap --help` output:

- First one is that argparse shows short and then long form of --help option, while powrap's arguments show long and then short, causing it to look like:

```
options:
  -h, --help      show this help message and exit
  --modified, -m  Use git to find modified files ...
  -C GIT_ROOT     To use with --modified to tell ...
  --quiet, -q     Do not show progress bar ...
  ...
```

- The other thing is that the exit code is set in multi-line string, but not actually printed as multi-line string. While it looks compacted and OK in the printed help message, I assume that's not the intended so I set RawTextHelpFormatter to make sure the formatting is honored.

```
exit code: 0:nothing to do 1:would rewrap 127:error running msgcat
```

Reviewed-on: #2
Co-authored-by: rffontenelle <rffontenelle@gmail.com>
Co-committed-by: rffontenelle <rffontenelle@gmail.com>

powrap/powrap.py

@@ -94,14 +94,15 @@ def parse_args():
     parser = argparse.ArgumentParser(
         prog="powrap",
         description="Ensure po files are using the standard gettext format",
+        formatter_class=argparse.RawTextHelpFormatter,
         epilog="""exit code:
     0:nothing to do
     1:would rewrap
   127:error running msgcat""",
     )
     parser.add_argument(
-        "--modified",
         "-m",
+        "--modified",
         action="store_true",
         help="Use git to find modified files instead of passing them as arguments.",
     )
@@ -114,11 +115,11 @@ def parse_args():
         default=Path.cwd(),
     )
     parser.add_argument(
-        "--quiet", "-q", action="store_true", help="Do not show progress bar."
+        "-q", "--quiet", action="store_true", help="Do not show progress bar."
     )
     parser.add_argument(
-        "--diff",
         "-d",
+        "--diff",
         action="store_true",
         help="Don't write the files back, just output a diff for each file on stdout "
         "(implies --check).",