fix: pretty-make should work without subcommand

[Awea]

We need to keep default behaviour pretty-make Makefile or at least display an helpfull error message: to produce a pretty help please use pretty-make pretty-help Makefile.

Read https://github.com/sharkdp/bat/tree/master/src/bin/bat sources to know how to handle subcommands allongs with default arg (pretty-make Makefile and pretty-make pretty-help Makefile.

[Awea]

Or it should display help from pretty-make --help