Provide public API for customizing output

[tomghuang]

@hzeba has provided a great example (format.zip), which shows how to customize glossary and error message output format. However, the example has to use several private functions (such as arg_cat_optionv() and arg_print_formatted_ds()) to customize the output. It means that Argtable3 doesn't provide enough public API for this purpose.

We need to carefully open several private functions, so that developers can have enough public interface to customize their output.

[jhzeba]

Here's a c++ wrapper if anybody's interested.