Default to the input glyph sequence for output

* Make out_glyph_sequence follow the source glyph sequence unless overridden

* Update relevant documentation in help

Author: pushfoo

fontknife/frontend/__main__.py

@@ -114,6 +114,11 @@ def main() -> int:
 
     # Generate requested output
     output_path, output_kwargs = get_output_path_and_args(raw_args_dict)
+
+    # default to using input glyph sequence if none specified
+    if output_kwargs.get('out_glyph_sequence', None) is None:
+        output_kwargs['out_glyph_sequence'] = source_kwargs['glyph_sequence']
+
     args.callback(font, output_path, output_kwargs)
 
     return 0

fontknife/frontend/arg_parsing.py

@@ -131,9 +131,12 @@ def cli_grapheme_arg(source: Optional[str]) -> Optional[Tuple[str, ...]]:
         'short_flag': '-g',
         'type': cli_grapheme_arg, 'default': None,
         'help': '''\
-            Manually override the order of the emitted glyphs.
+            This defaults to the value of the input glyph sequence.
 
-            Be careful when using this.
+            Pass it to manually override the order of the emitted glyphs.
+
+            Be careful when using this as OS-specific escaping can come into
+            play.
         '''
     },
 }