V-DataTable Formatting function

[ghost]

I would attempt to do it myself and submit a pull request but ill probably stuff it up :P. But with the headers object in the v-data-table, adding the support of a formatter function that is used to know what value to render. Situation of it being useful is a table that has many Date objects and you currently have to slot every single one to format it as a date.

While i know i could modify the actual value in the object to be the formatted one, this breaks sorting and other features, being able to just add a function to the headers object would save alot. Just an idea