Implement common flags and options

[rsore]

A detail that increases verbosity of the library API is defining custom flags and options. There are a lot of common flags and options that programs use, such as --verbose and --config. The library should provide many of these so the users don't need to make custom types for them. Here is a proposed implementation:

namespace CLArgs::CommonFlags
{
    template <::CLArgs::StringLiteral Description = "Enable verbose output">
    using Verbose = ::CLArgs::Flags<"--verbose,-v", Description>;
}

namespace CLArgs::CommonOptions
{
    template <::CLArgs::StringLiteral Description = "Specify configuration file">
    using ConfigurationFile= ::CLArgs::Option<"--configuration,--config,-c", "<filepath>", Description, std::filesystem::path>;
}

Instead of defining their own custom types for these common flags and options, they can simply use these predefined types, optionally providing a custom description more relevant to their specific program:

using Parser = CLArgs::Parser<CLArgs::CommonFlags::Verbose,
                              CLArgs::CommonOptions::Configuration<"My custom description">>;

This would be a very simple improvement to the library, but a very valuable addition.