Bugfix issues #39, #40 and #41

[pamiel]

This PR addresses the following issues:

• -t option used on an unknown chart #39: check is now performed when using the -t or -x options: if one of the given chart names is an unknown sub-chart name, then Spray exists with error
• Fetch of tgz file not working on Windows #40: Fetching of a tgz in a repo is now possible on Windows
• {{ .File.Get }} not working if no {{ pick (.Files.Get) }} #41: using #! {{ .File.Get }} in the values.yaml of an umbrella Chart without having any #! {{ pick (.Files.Get ) }} in the same file is now working