Add go-getter support to load base helmfiles

[jonasrutishauser]

Resolves #1193

[mumoshu]

@jonasrutishauser Hi! This looks good. Just confirming, but how did you test this? Is this tested to work both with or without the use of go-getter style url in a bases entry?

[jonasrutishauser]

I tested it manually (simple Helmfile) with and without go-getter style url in bases.
I even tested my tests by running them against the code without my changes.

[mumoshu]

I see! Let's go ahead. Thanks for your contribution @jonasrutishauser ☺️

[mumoshu]

@jonasrutishauser Ah, would you mind adding some documentation about this feature under https://github.com/roboll/helmfile/blob/master/docs/writing-helmfile.md#layering-state-files so that it's more discoverable? Thanks!