Add HspecOpenTest command and default mapping

[alx741]

So here is a little convenience: When working in a file Foo.hs now is possible to do ghs (the default mapping) to jump to FooSpec.hs.

This defines a command HspecOpenTest and a default mapping for it ghs.

It allows some configuration as well:

• Disable the default mapping (ghs)
• Add directories where to look for the tests (recursively)
• Test files prefix
• Test files suffix (default is Spec)