Only expand selected rules

[nichtich]

Expanding all rules can result in a very complex diagram. An option to select which rules to expand (and/or how many levels of recursion) would help. How about this:

• change the option --expand to a numeric argument with 0 as default and all alias for a very large number
• change probably this to decrease expand for each recursion.

This way callers can decide the depth of expansion and get arbitrary selections of the grammar when combined with --start.

[hildjj]

If expand starts to require a parameter, that would be a breaking change. If expand takes an optional parameter, it would also be a breaking change to peggy-tracks -e test.peggy. We could add a (somewhat redundant) new option called "depth" or "maxDepth" that would require an integer parameter. 0, the default, would mean no expansion. --expand would set depth to Infinity.