loader Support for Compressed Dumps

[dotmanila]

Feature Request

Is your feature request related to a problem? Please describe:
When the source data is generated with mydumper -c, loader cannot load it.

Describe the feature you'd like:
Support for compressed dumps.

Describe alternatives you've considered:
Uncompress manually or dump without compression.

Teachability, Documentation, Adoption, Migration Strategy:
All around compression uses for large datasets being migrated.

2019/05/19 03:50:25 loader.go:524: [info] [loader] prepare takes 0.000139 seconds 2019/05/19 03:50:25 loader.go:332: [error] [loader] scan dir[backup/] failed, err[invalid mydumper files for there are no -schema-create.sqlfiles found] 2019/05/19 03:50:25 main.go:84: [fatal] /home/jenkins/workspace/build_tidb_enterprise_tools_master/go/src/github.com/pingcap/tidb-enterprise-tools/loader/loader.go:429: invalid mydumper files for there are no-schema-create.sql files found /home/jenkins/workspace/build_tidb_enterprise_tools_master/go/src/github.com/pingcap/tidb-enterprise-tools/loader/loader.go:333: