Join/Spectate support, and bugfixes.

String-ish code in the extension was rewritten from char* to std::string.
oh, and yes, now we support join and spectate.
timestamp functions now can treat GM timestamps (you can still switch the old behavior by setting is_unix flag to true)
Linux version now doesn't rely on a shared Discord-RPC library, it's now statically linked into libnekopresence.so

as always, .yymp is for GMS 2, .gmez is for GM:S 1.4, the only difference is Linux libs.

neko neko nya nya~