Add label attribute to `projectable_col`

[kinto-b]

Possibly introduce labelled as a dependency to get labeling working.

Ideally we want to have the prj_tbl_*() functions working in such a way that:

• The rows column of prj_tbl_summarise() is filled with value labels of the row columns by default
• If the columns of prj_tbl_summarise() are stipulated using the shortcut approach of supplying a variable name only, the output columns are labelled with the value labels of that variable