Introduce special mark for cross config references

[patrickpichler]

As of right now we cannot really distinguish between simple values and cross config
references. The only way to see if a value is a cross reference is by checking them
to end with either .id or .name. With this approach every value ending with either
of those will get interpreted as a reference.

This approach right now does not scale.

Once we decide to add more and more cross references options, this will get more
and more complicated.

I propose to use some special format to mark cross config references.
Something like $.

So if we now want to cross reference the id of another config we simply write
$/other/dashboard.id or $other/dashboard.id.

That way we make cross references explicit.

As this would be a breaking change, we should think about introducing a helper
command for migrating to this new convention.

It would also make sense to switch all paths to an os interdependent format (#21)
while doing this, as this would also break compatibility.

[tatomir146]

Just a though: would it be possible to use go templates for this? We already provide environment variables {{ .Env.MyVar}}. Wouldn't it be nice to have something like this {{ .Var.project.dashboard.whatever}}?