Peter M. Groen / cxxopts

These were detected using -fsanitize=undefined parsing values equal to
INT_MAX and INT_MIN.

Fixes #169. It seems reasonable to throw an exception when an attempt is
made to parse into positional parameters that don't exist.

Fixes #124.

Fixes #143. There is no reason to make the tests optional.

Fixes #121. Removes the unused `ParseResult::get_option`.

Fixes #115. Read the version number out of `cxxopts.hpp` instead of
having to duplicate it in CMakeLists.txt.

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 #101. Allows leading zeroes in the integer parser.

Fixes #86. Rename some variables so that the overly strict shadows warning
on GCC 4.9 is not triggered.

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.

add booleans into the changelog and the readme

This is far from ideal, but it's the first step in moving the parse
result to an immutable object so that the parser can be reused.