Fixes#83.
Adds an overload for positional arguments taking a
`std::initializer_list`. When using an `initializer_list` with one
argument, the function call is ambiguous matching both `std::string`
and `std::vector<std::string>`.
Fixes#84. Implicit values are not handled very well. For example:
--foo false true
--foo --bar
In the first, `false` is an argument to `--foo`, and then `true` is a
positional argument. In the second, because of the hyphen in `--bar`, `--foo`
is parsed with its implicit value. This seems inconsistent and unintuitive.
Better is that implicit values *never* consume the next parameter to be
completely consistent. This means that values with an implicit parameter
*must* be specified using the `--option=value` form.
