datapkg_write() - support writing from in-memory Data Package

[danfowler]

Having created a Data Package using datapkg_read(), I can update its metadata through simple assignment. However, there's currently no way to write that an in-memory Data Package to disk. datapkg_write() appears to only support writing data from data frames.

[dy-kim]

I'd like to make an additional remark on 'writing Data Package', for what it's worth. I think when there are little changes on an in-memory Data Package compared to an original Data Package on disk, it would be much efficient to 'update' the diff rather than rewrite whole csv file.