New item helper that knows about filters

[dgm]

When a filter is in effect, it would be nice to have a helper that knows about the filter params, such that the filter is passed into the new form. the new form controller could then parse the filter params and initialize any fields to match the filter. Related to the example in issue #6, this would mean that if a filter was set on the transaction_type column with a value of "cash", clicking a "new transaction" link would go to a form with the transaction type already set to "cash"

[dgm]

Something like:

<%= link_to_with_filter(label, path, options) %>