Feature requests: piping into stdout, arbitrary entries

[iaaka]

Hi, thanks for a great tool! Few new features I would love to see:

1. option to write output (at least for csv files) into stdout.
2. arbitrary entry (not limited to var/obs): one may have something usefull in obsm (UMAP), plus you'll be able to read looms (any reasonable h5) in this way.
3. add ls command that will print tree like structure of h5 file: again a way to extend support beyond h5ads and obs/vars