Add configuration for forcing case-sensitive key/tag matches in decoding.

[gavbaa]

This pull request proposes adding a new configuration option: CaseSensitiveKeys. If true, this would only allow for exact matches on struct Field/Tag names, whereas if false it would use the current behavior of doing Unicode code-folding on the key maps.

Example use case: The Experience API standard (xAPI, detailed here: https://github.com/adlnet/xAPI-Spec/blob/master/xAPI-Data.md#lrs-requirements ), which is defined on top of JSON, requires that "The LRS MUST reject Statements...where the case of a key does not match the case specified in this specification." Someone bringing in the JSON data and wanting to use mapstructure to copy it into appropriately defined structs has to be sure that the "id" key is "id" and not "ID" in order to validate within the specification.

Good test coverage included in pull request.

There's also a tiny performance perk by not having to iterate the map! :)

[abemedia]

@mitchellh this can probably be closed now since #250 is merged.

[mitchellh]

Good call thanks!