Add ebur128_reset to reset measurements

[renaud-sc]

Useful when used as a Live Loudness Meter

Check jiixyj/libebur128#100

[rsp4jack]

you should add ebur128_clear to ebur128.def

[luzpaz]

any traction on this ?

[bmatherly]

This pull request is not a priority in my opinion since a reset can be achieved by calling destroy/init. In fact, that is exactly what I do in MLT for live applications: https://github.com/mltframework/mlt/blob/master/src/modules/plus/filter_dynamic_loudness.c#L53