Add flush method to LZ4FrameFile

[pedrovhb]

Hello! First off, thanks for the library, it's been quite handy 🙂

The LZ4FrameFile abstraction is somewhat leaky/surprising in that it offers a flush method, as expected of a file, but it doesn't actually flush compressed data as one may expect. The method is just inherited from io.BufferedIOBase and doesn't have special logic to handle LZ4.

This PR introduces the missing logic so that calling flush correctly ensures the frame is completed and flushed to the underlying fp.

I'm not particularly familiar with the concepts behind compression algorithms, so while this really just deals with the file interface, works on the cases I've tried, and passes tests, please do look at it critically and let me know if I missed something!

[jonathanunderwood]

Thanks for this, I really appreciate the time you spent on it.