Add support for TartanLlama's `tl::optional`

[vittorioromeo]

This PR adds support for tl::optional to scelta. This excellent optional implementation provides useful continuations. E.g.

tl::optional<image> get_cute_cat (const image& img) {
    return crop_to_cat(img)
           .and_then(add_bow_tie)
           .and_then(make_eyes_sparkle)
           .map(make_smaller)
           .map(add_rainbow);
}

tl::optional support is enabled if __has_include(<tl/optional.hpp>). When enabled, the SCELTA_SUPPORT_OPTIONAL_TL is defined.

Automatic detection of tl::optional can be disabled by defining SCELTA_SUPPORT_OPTIONAL_TL_DISABLE.