Allow exclusion of fields about snapper

[theramis]

Hey @colindooley11,

Sorry about the super late reply. I've had this kind of a feature in mind for a while but didn't feel like it needed to be Snapper as like you mentioned the user can easily pre-process their own object.
I saw the PR you made #78 and the implementation is quite separate from Snapper.

Or different thing to what I am after it's more like ignoring the value of a field or defaulting a a changeable field to a fixed value....

This is what I originally had in mind but I decided to hold off on it for now for two reasons.

1. I haven't seen enough demand for this feature
2. I would like Snapper to encourage people to make deterministic tests. If there are parts of the snapshot changing, it violates that and can give a false sense of security to ones tests.

[gogulsekar]

+1