The previous commit had the archive condition on the wrong place,
resulting that todo.txt files were not written out at all if archiving
was disabled.

Also introduce the (overall) -a flag to disable auto archiving. -a comes
from the original todo.txt CLI.

It would have been nicer to have DoCommand support the -a flag, however,
DoCommand doesn't know anything about archiving and I would like to keep
it that way.

Addresses issue #42.