Prefix command

[henryiii]

Add support for a prefix command, where a different app can be called after parsing. This is almost already supported; the problem is that you can't do the following:

./my_app --my_app_flag another_command --another_command_flag --my_app_flag

The final --my_app_flag will be parsed and used with CLI11 before returning the unparsed arguments.

You could use subcommands to solve this problem already, but then you would have to know the another_command beforehand.

See https://stackoverflow.com/questions/37996617/create-a-prefix-command-with-boost-program-options

[henryiii]

Added.