Fixes#344 by allowing explicit selectors to be specified for composed helmfiles using the following structure
```yaml
helmfiles:
- path: helmfile.d/a*.yaml
selectors:
- name=prometheus
- name!=zipkin
- helmfile.d/b*.yaml
- path: helmfile.d/c*.yaml
selectors: {}
```
2 modes here :
* legacy mode when no the env var HELMFILE_EXPERIMENTAL is not set to true
* no selector : inherit from the command line.
* selector: is specified then it is used (an emty means no inheritance from command line and take everything).
* experimental when the env var HELMFILE_EXPERIMENTAL=true
* no selector : nothing is inherited from the command line so use all releases.
* selector: is specified then it is used (an emty means no inheritance from command line and take everything).
sgandon