log,errors: Option to compress rolled log files

[mholt]

Lumberjack recently added the ability to compress rotated log files using gzip. It'd be useful if we could expose that feature in the Caddyfile with a rotation directive similar to rotate_size (which would be before compression), called rotate_gzip or rotate_compress.

The only compression option is gzip. This is enabled by setting the Compress field to true.

The log and errors directives will use this. We do not need to change the core process log's settings; right now there's no way to configure that log anyway (other than its filename).

This enhancement is very simple to implement, so I am reserving this for newcomers to the project!

NOTE: This will require updating the vendored dependency. We use gvt for this.