Getnative

+ Fix R60 support

+ LSMASH is the default source now (AkarinVS)
+ Added bicubic b=0, c=0.75 as predefined scaler (AkarinVS)
+ Updated README

+ R60 Support 19

+ Let vpy_source_filter work with VS R55+ 18

+ Add compatiblity with VS 55

**Breaking changes**
+ Make the module compatible with async discord bots
+ getnative() function is now async
+ getnative() function returns now 3 values: best value string, pyplot and class object of getnative class

Example of how to use it in a bot:
https://github.com/Infiziert90/Tsuzuru-Bot/blob/master/commands/vapoursynth_commands.py#L165