Add a value hook func

[camdencheek]

Overview

This commit adds a third hook func type which has access to the reflect.Value representations of both the from and the to objects. This gives all the same power of the existing hook functions (types, kinds, and data can all be retrieved from values), but with the additional benefit of being able to modify the to object.

Change scope

This change is largely backwards compatible. The only change of the public API (other than adding a new hook type) is a change in the signature of DecodeHookExec. If that's a sticking point, I think there may be a way around it, but I haven't looked closely at it.

Motivation

Exposing the value of to opens up some really powerful patterns such as:

• Recursively decoding structs to maps (Encode: DecodeHook not called for struct fields #166, Nested map[string]interface{} not decoded #124). This PR includes a hook func that does this
• Setting the concrete struct type behind a destination interface before continuing to decode
• Setting defaults with tags (Default value #111) or with a Defaulter interface
• Implement a hook for respecting Unmarshaler interfaces (Implement JSON like unmarshaling interface #115)

Overall, I think if this is done right, it could really open a lot of possibility for more powerful hook funcs. The API works well, but I plan to play around with it a bit more to make sure it's as flexible as possible. I'm obviously open to any feedback

[smyrman]

Alternate suggestion in #192

[mitchellh]

I like this! I like that it fits into the existing style of decoder hooks.

As much as I'd rather not break BC, the DecodeHookExec function I've never seen used and probably shouldn't have been publicly exposed in the first place. I think I only did that for testing helper purposes. I'd be okay with breaking API in this PR for that specific API.

I'll merge this as-is, but let me know if you wanted to make other changes to it. Thank you for the tests.