a struct field not being found should be an ErrNotFound error

Adding ValueTransformationHookFn (#10)

* Adding a GetHook.

* Allow GetHook to run on the top level object being pointed on.

* Disallow GetHook to run on the top level object being pointed on.

* Rename GetHook to ValueTransormationHook and update doc comments.

* Fix a missed rename to ValueTransformationHook in an error message.

* Applying comment suggestions.

Allow commas in struct tags, simply ignore everything after the comma (…

…#8)

Return descriptive and comparable errors (#6)

* Return descriptive and comparable errors

* Make new error handling work pre-go1.13

The code has been refactored that pre-go1.13, the behavior is
identical to the way it was before. Tested on go1.15, 1.12
(reverted the updated go.mod to its original state), and 1.7.6

* Revert "Make new error handling work pre-go1.13"

This reverts commit 1f54a4f.

Add a license (#5)

MIT per Seth Vargo, Fixes #4