While updating the `history` function to use `argparse` I realized it is
useful to define an option that can be used in three ways. First by
using the short flag; e.g., `-n NNN`. Second by using the long flag;
e.g., `--max NNN`. Third, as an implicit int flag; e.g., `-NNN`. This
use case is now supported by a spec of the form `n#max`.
This implements support for numeric flags without an associated short or
long flag name. This pattern is used by many commands. For example `head
-3 /a/file` to emit the first three lines of the file.
Fixes#4214
We've needed a fishy way to parse flags and arguments given to scripts
and functions for a very long time. In particular a manner that provides
the same behavior implemented by builtin commands. The long term goal is
to support DocOpt. But since it is unclear when that will happen so this
implements a `argparse` command. So named as homage to the excellent
Python module of the same name.
Fixes#4190
