Explain usage of `--start` in documentation better and to mention that `now` is a time option

[silverhook]

I have 1000+ tasks (in a tree structure) and each month/week I add start dates only to those that I actually want to work on this period.

Currently this use case is not supported by todoman. It does have --startable, but that is quite a different beast.

I propose to have a --started flag to only show tasks that have their start date on today or an earlier day.

Since my proposal is actually achievable (see next comment), I suggest it is better documented instead, so it is easier to find for the user.

[silverhook]

After some trial-and-error, I found this to do what I want:

todo list --start before now

But I could not figure this out from the documentation alone, because now is not mentioned as an option. In fact, the --start flag itself is not described either.