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