Change `batch` command to take a source folder name

[richardfearn]

Originally done as a temporary change on a branch called split, that changed the batch command into a split command:

'batch' takes articles from the "Read Later" folder and batches them into folders named after today's date - yyyy-mm-dd-{a,b,...}.

'split' takes articles from a specified folder X and batches them into folders named X-{a,b,...}.