Enable dateField to put a timestamp with a local time zone

[mnacamura]

I noticed that dateField enforces putting timestamps with UTC time. However, one may want to use a local time offset; at least I want to do so ;)
This PR achieves it.

[jaspervdj]

This looks more or less good. Suggestions:

1. I would put the TimeLocale as Maybe TimeLocale in FeedConfiguration. Then, I would also add a defaultFeedConfiguration which sets this to Nothing (yes, I'm aware that this is a breaking change but it should be easy to fix for people).

2. Remove the DateFormat from the dateField function and just use the default one there.

3. Add a record which bundles up the settings for dateFieldWith:

   data DateFieldConfiguration = DateFieldConfiguration
       { dfcOutputTimeLocale :: TimeLocale
       , dfcDateFormat :: DateFormat
       }
   

I hope these changes make sense.