This is fix for formatting partial references formatted in invalid way.

Say we have:

data = {"FOO": "bar", "BAZ": "{FOO}", "X": "{Y[z]}"}

result of acre.compute() should ideally be:

>>> acre.compute(data)
{'FOO': 'bar', 'BAZ': 'bar', 'X': '{Y[z]}'}

Although X can be legitimate value in environment variable, formatting it will fail because it triggers python formatting function expecting it references list with z as indice and indices must be ints.

Solution is to try the original way and if it fails, format each reference separately and if one of them fails, return unformatted value.

Resolves #7