Feature request: support store_as gzip

address #16
This patch adds support for gzip-compression of data before storing it.
Yes, only gzip.

In my ssl+kerberos+httpfs environment, this patch works fine.
I'm willing to change the parameter name.
I referred to `out_s3` https://github.com/fluent/fluent-plugin-s3/blob/master/lib/fluent/plugin/out_s3.rb#L30
Is `compress` better than `store_as`?