sas column labels -> parquet metadata

[schmeisers]

When converting to parquet, it would be pretty handy if the sas column labels were written to the column key:value metadata in the resulting parquet file.

[curtisalexander]

This is a good idea and something I would consider as part of next release. My time spent working on this software is quite limited and so it may be some time before I get to this work and the next release. I would accept a pull request if you would like to see the feature added more quickly.