Add start/end of previous working day date aliases

For my daily stand-up meeting, I use the `totals.py` script to quickly generate a summary of what I did yesterday (`timew totals yesterday for 1d`). Unfortunately, come Monday, I can't use this command to show what I did on Friday, so I end up doing `timew totals for 1d` which is substantially more annoying.

This could be solved by adding start/end of previous working day aliases (`sopwd`/`eopwd`) in the vein of the existing `so*` aliases: `timew totals sopwd for 1d`

(Obviously it would also make sense to add the current/next working day variants at the same time.)