Support for a minimum number of repeated arguments

Whether `positional` or `option` arguments that are of type `Vec` are always treated as "0 or more", with no way to specify "one or more", or "two or more".

This is similar to issue #13, but a more general case.

I can make two proposals here:

**Proposal 1:**

add an attribute such as `#[argh(option, required)]`, which forces _some_ value to be present for vec-types.

**Proposal 2:**

add an attribute such as `#[argh(option, required(2+)]` or `#[argh(option, required="2+")]`

Help text would need to be adjusted accordingly as well, such as:
```
Usage: cmdname --option --option [--option ...]

Options:
--option some option (2 or more required)
```