parse(): support both `const char **argv` and `char **argv` interface version

[GerHobbelt]

parse(): support both const char **argv and char **argv interface version.

Some codebase have argv as const char **argv or const char *argv[] while others have it without the const modifier. This takes care of that and makes clipp a drop-in for both styles.

(cherrypicked off https://github.com/GerHobbelt/clipp master branch; original commit tested with latest MSVC2019)

[chirsz-ever]

Maybe changing char* argv[] to const char* const* argv is enough. Non-const arguments would be auto converted to const type.