class GeneralArgsParser

A command line arguments parser.

Background: The fxl::CommandLine class is designed to be either a global or

passed around, and code can query it for random switch strings as it feels

necessary. In the fxl::CommandLine model, the set of command-line switches

isn't known in advance. This means that it can't generate help or validate

the switches. It also can't support switches with values that don't use an

equal sign ("-c foo") because it can't disambiguate whether "foo" is a value

for the "-c" switch or a standalone parameter. fxl::CommandLine also

supports modification and generation of new command lines.

In contrast, this command-line parser requires registration of all switches

in advance. It can then validate inputs, generate help, and accept more

flexible value formatting. This class is designed exclusively as a parser

which makes it simpler.

The command line args parser has two layers. The lower "general" layer is a

concrete class that parses the command line and calls callbacks for

switches that have been registered.

On top of this is the ArgsParser which is a template that fills

a struct with registered options. This is what most code will want, but

you can still register custom callbacks for more complex behavior.