This basically allows to define both `releases` and `helmfiles` within a helmfile.yaml, so that you can start using sub-helmfiles easily, by extracting only reused releases.
Resolves#445
* Improve code organization
To make sure it is still readable after upcoming changes to helmfile
* feat: `helmfile deps` to update dependencies of all the local charts
Resolves#450
* feat: helmfile updates repos and build deps by default
But not update deps. Use `helmfile deps` to update deps, and provide `--skip-deps` to skip updating repos and builds deps in sync/diff/apply/template
Resolves#415
* Improve integration test coverage
Patrick Valsecchi
KUOKA Yusuke