docs: GNU-style help formatting for option args.

Derive `GnuStyleHelpFormatter` from `argparse.HelpFormatter` to format
option-arg pairs in the help message according to GNU style:
https://www.gnu.org/software/libc/manual/html_node/Argument-Syntax.html.

For example,

```
  -p, --path=PATH      		Help string for a path option
```

instead of the default:

```
  -p PATH, --path PATH 		Help string for a path option
```

src/tartex/_parse_args.py

@@ -82,13 +82,52 @@ def __call__(self, parser, namespace, values, option_string=None):
         parser.exit()
 
 
+class GnuStyleHelpFormatter(argparse.HelpFormatter):
+
+    """
+    Format help string in GNU style, i.e.
+
+    * `-s, --long           Help string for long`
+      for an action that takes no argument
+    * `-s, --long=LONG      Help string for long`
+      for an action that requires an argument
+    """
+
+    def _format_action_invocation(self, action):
+        if not action.option_strings:
+            default = self._get_default_metavar_for_positional(action)
+            (metavar,) = self._metavar_formatter(action, default)(1)
+            return metavar
+
+        parts = []
+
+        # if the Optional doesn't take a value, format is:
+        #    -s, --long
+        if action.nargs == 0:
+            parts.extend(action.option_strings)
+
+        # if the Optional takes a value, format is:
+        #    -s, --long=ARGS
+        else:
+            default = self._get_default_metavar_for_optional(action)
+            args_string = self._format_args(action, default)
+            for option_string in action.option_strings:
+                if len(option_string.lstrip("-")) == 1:  # short form
+                    parts.append(f"{option_string}")
+                else:  # long form
+                    parts.append(f"{option_string}={args_string}")
+
+        return ", ".join(parts)
+
+
 def parse_args(args):
     """Set up argparse options and parse input args accordingly"""
     parser = argparse.ArgumentParser(
         description=(
             "Build a tarball including all source files needed to compile your"
             f" LaTeX project (version {__version__})."
         ),
+        formatter_class=GnuStyleHelpFormatter,
         usage="%(prog)s [options] filename",
     )