Plush command-line option parser

[maximecb]

ZetaVM has an option parser, written in C++, which was contributed by ashwanidausodia. This option parser handles the command-line options of the VM itself.

However, programs running on ZetaVM can receive their own options, eg:

# Options after the double dash are passed to the program being run by ZetaVM
./zeta myprogram -- opt1 ... optN

Currently, the options passed to programs running on ZetaVM are handled in an ad-hoc manner. See this program for an example.

It would be desirable for us to provide an std/options library to parse command-line arguments. This library should be written in Plush. We may want to try and have an API similar to the C++ option parser in ZetaVM, for consistency, though this isn't strictly necessary.

Note: there are already a number of useful functions in the std/string and std/array libraries which may be useful in implementing the option parser.