Allow custom post metadata casting

[Josh Duff]

@spacetag got an example of using lists of post tags working, but had to add custom code to split the tag string into an array: https://github.com/TehShrike/noddity/pull/18/files#diff-e6c5621cc8e5d3b3e528876df143ab48R59

I feel like Noddity users should be able to set custom casting options to be passed to the weak-type-wizard. They would need to be set in the config.js file.

Discussion point: how would this be supplied to services like rssaas?

[Josh Duff]

We've been getting by with splitting up category lists in the template code so far with Kayser Commentary.