Made parameters an interface

[oxtoacart]

@Knetic Thanks for govaluate, I've found it to be very useful. One problem I had is that specifying parameters as a map[string]interface{} is a little inconvenient because I have them in a different data structure. I can convert by copying them, but that gets expensive for what I'm doing.

This PR provides a new Eval method that's like the existing Evaluate except that it takes a new interface Parameters. This allows me to pass in a different data structure that just provides a Get method.

[oxtoacart]

Instead of sanitizing parameters up-front, we just wrap the supplied parameters with SanitizedParameters that does the validation at parameter access time.

[Knetic]

I like these changes. Even if a user's structure doesn't have a method that exactly matches the signature of Get(string)(interface{}, error), this gives them a way to wrap their structures (like how MapParameters does) so that it works. This seems like a reasonable way to offer users with other data structures a way around copying.

I'll follow up with a couple minor edits for perf reasons (I noted this increased benchmark times by a flat ~50ns, which isn't huge, but is avoidable), but this looks good to me. Thanks for the PR! I'm glad the library is useful to you!

[oxtoacart]

Thanks @Knetic !