Allow disabling teNOR's optimisation passes

[ISSOtm]

This should be relatively easy:

1. Add a new option (in main.rs); I'm thinking something like -O -trim_patterns,+prune_instruments. (-O is a common "optimisation" flag, e.g. in C compilers.) The argument would be a comma-separated list of pass toggles, with the + or - prefix indicating whether to enable or disable the pass.
2. For each pass, if it's disabled, then replace it with a dummy operation that doesn't perform any optimisations. (For example, the "row overlap" pass would instead return some arbitrary order, and no overlap.)

Rationale

This would be useful for people doing tricks with the song data (e.g. using routines to manipulate the song pointer).