Support --columns in CLI

[will-moore]

See ome/omero-parade#47
Would be nice to be able to use the --columns argument in the CLI

Need to add this to:
https://github.com/ome/omero-metadata/blob/master/src/omero_cli_metadata.py#L480

Also add this to message at
$ omero metadata populate --help

[sbesson]

For anyone interested in tackling this feature, a few pointers:

• adding a new CLI argument can be done as in https://github.com/ome/omero-metadata/blob/v0.2.2/src/omero_cli_metadata.py#L205. I assume this only applies to the populate --context csv use case.
• as mentioned above https://github.com/ome/omero-metadata/blob/v0.2.2/src/omero_cli_metadata.py#L480 allows to call the relevant API
• integration tests should probably be added to metadatatest and/or clitest to cover the semantics of the columns argument