Formatter: enable to select multiple identifiers and save records as MARCXML

[pkiraly]

It is possible to download identifiers matching to a query. With Solr API one can download the records as JSON, but it is not possible to extract records as MARC XML. The formatter supports --id parameters that prints out a single record to standard output. For the purpose of the Cultural Analytics course we need these features: select multiple records by their identifiers, and export them into a MARCXML file. On the interface level we need am --ids parameter that accepts a comma separated list. We should also enable --format parameter to support xml.