Store all document metadata

[talolard]

Currently we only store the text field of the document, but a user would probbly want all of the metadata that came with it. Also, in the future we'd probably want to add search on it.

The offending code is here https://github.com/LightTag/ylabel/blob/2a7d4b98a3e52085cbcf1a6516758c1805e13b4c/src/data/components/input/fileInput.js#L62

And it's also a perofmrance hit because we're doing an extra loop on the dataset.

The problem is, we need to know the name of the field that has the text. For know, I just shove it into content, but maybe better solution is to store the name of the text field somehwere and just use it when we build the postings list and the example. The index itself doesn't know or care about this field