When user code is working on a 'const char**' they have to do
a 'const_cast' in order to pass argv into the cxxopts::parse
function.
The rationale behind this commit is to accept both 'char**'
and 'const char**', but internally use 'const char**' due to
const correctness.
Fixes#109. Although we always used numbers in the ASCII range, it was
in a `size_t`, leading to a warning about possible loss of data on some
compilers.
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.
