Add an exit/quit command

The previous method of using C-d to exit the prompt will still work, of
course.

Close #21
Add help text for "eventually"

Also clean up help text for "add" slightly, because it was too long.
Implement a -n flag to only show n tasks

Note that passing -n=-1 will show all tasks.

Close #20
Add optional maxItems argument to "list"
Use standard bracket argument documentation

Previously, the built-in helptext used square brackets to denote any
argument, but the general UNIX standard is to use square brackets to
denote optional arguments. This commit fixes that.