Enhance FilesFromDisk (close #331)

[mholt]

Now permit suffixing root on disk with separator to enumerate only
its contents and not to create that actual file in archive.

And attempt improving Windows compatibility. I don't have Windows handy so this'll have to be CI-based feedback.

@nicola-spb could you give this a try?

[nicola-spb]

@mholt I am working in Linux too :(

[mholt]

That's fine, I mostly want you to test the feature you requested.

[nicola-spb]

Yes it's work on Ubuntu.

Tested it:

files, err := archiver.FilesFromDisk(nil, map[string]string{
	"/path/on/disk/folder/": "",  // adding files to root in archive
})

files, err := archiver.FilesFromDisk(nil, map[string]string{
	"/path/on/disk/folder": "",  // adding files to "folder" in archive
})

[mholt]

Perfect, thanks for verifying it does what you need!

I'll clean up the implementation and get them working on Windows. I might also work on the dot (.) situation but how complex it is depends whether it goes in this PR.