Add support for compression levels, closes #1042

[bobrik]

This makes it possible to set compression level for gzip:

config.Producer.CompressionLevel = flate.BestCompression

There are no other compression codecs that support levels,
but the code is generic to easily add other codecs with
numeric levels.

Default compression level tracks what Go libraries
set as the default level.

[bobrik]

I started with *int for compression level with nil being the default, but that turned out to be ugly, because you can't take address of a constant like flat.BestCompression.

[eapache]

I'm not sure if it's worth throwing any checks into Validate() or if we should just rely on the libraries we call during encode()?

[bobrik]

@eapache I added another commit with validation check:

2018/02/12 19:51:34 Error creating producer: kafka: invalid configuration (gzip compression does not work with level 12: gzip: invalid compression level: 12)

[eapache]

CI is going to fail because you're not accounting for the default value the same way in Validate as in encode

[bobrik]

I just added the check.

[eapache]

Thanks!