Add exporter option to sort exported items

[nichtich]

To sort items by a given field, e.g. sort by field creator.name or to sort reverse by numeric field size:

catmandu convert JSON to JSON --sort creator.name
catmandu convert JSON to JSON --rnsort size

I'd prefer four options sort, nsort, rsort, nrsort instead of sort, numerical and reverse but names can be discussed. Sorting would always imply buffering all items before export.

[phochste]

Can in this case the sort in the to_array() feature be deleted? It would be a much cleaner to do something like:

 Catmandu->importer(...)->select(...)->sort(...)->to_array;

then

Catmandu->importer(...)->select(...)->to_array(sort => ....);

[nichtich]

yes both features should be discussed together.

[nichtich]

PR #161 implements $iterator->sort(...) without extended sorting options. I am not sure how to best express these options in the method and in the command line client. Possible options:

• reverse
• numeric
• key (can be code reference to calculate key for Schwartzian transform, e.g. for keys expressed in fix path language)

In the CLI I'd prefer short names, such as --rsort ..., this could be alias to --reverse 1 --sort ..., right?