This change enhances helmfile to accept terraform-module-like URLs in nested state files a.k.a sub-helmfiles.
```yaml
helmfiles:
- # Terraform-module-like URL for importing a remote directory and use a file in it as a nested-state file
# The nested-state file is locally checked-out along with the remote directory containing it.
# Therefore all the local paths in the file are resolved relative to the file
path: git::https://github.com/cloudposse/helmfiles.git@releases/kiam.yaml?ref=0.40.0
```
The URL isn't equivalent to terraform module sources. The difference is that we use `@` to distinguish between (1) the path to the repository and directory containing the state file and (2) the path to the state file being loaded. This distinction provides us enough fleibiity to instruct helmfile to check-out necessary and sufficient directory to make the state file works.
Under the hood, it uses [hashicorp/go-getter](https://github.com/hashicorp/go-getter), that is used for [terraform module sources](https://www.terraform.io/docs/modules/sources.html) as well.
Only the git provider without authentication like git-credentials helper is tested. But theoretically any go-getter providers should work. Please feel free to test the provider of your choice and contribute documentation or instruction to use it :)
Resolves#347
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).
bitsofinfo
KUOKA Yusuke
sgandon