Compress output before storing it to cache, if output compression is enabled

Based on PR #964
Use config_item() in CI_Output::__construct()
Merge changes from develop
Merge branch 'develop' into feature/output_compressed_cache
Merge branch 'develop' into feature/output_compressed_cache
Merge branch 'develop' into feature/output_compressed_cache
Merge branch 'develop' into feature/output_compressed_cache