Add ability to use go-getter for fetching remote manifests directory as chart

This, in combination with #1172, allows you to use `go-getter`-supported URL for K8s manifests on `chart`, so that Helmfile automatically fetches it and then turning it into a temporary local chart, which is then installed by Helmfile as similar as standard Helm charts.

An example usecase of this is to install cert-manager CRDs which is distributed separately from the chart:

```
releases:
- name: cert-manager-crds
  chart: git::http://github.com/jetstack/cert-manager.git@deploy/crds?ref=v0.15.2
```

I'm adding this based on discussion with @lukasmrtvy. He was trying to install cert-manager and prometheus-opreator with Helmfile, and this combined with #1373 should do the job. Thanks for the input!

Author: mumoshu