Support multiple indexes

[mattgodbolt]

For some data it's nice to be able to have multiple indices. For example, apache log files might want an index on the time, and one on the page name.

The current zindex command-line parser (TCLAP) isn't conducive to supporting all the per-index configurations that might be needed, so either a whole new syntax may be required, or else some other way of supplying an index configuration.

Maybe YAML, or JSON? If we need JSON anyway, that might be an option.

Maybe something like:

{
 "default": { "regex": "'\\[([^]]+)\\]'", "unique": false, "numeric": false },
 "page": { "regex": "GET|POST ([^ ]+)"
}

(uniqueness and numericness defaulting to zero, shown above for exposition only.

[mattgodbolt]

Thanks to @andreweskeclarke this is now done!