Trash permitted input keys

It is often useful to access a list of Trash's permitted input keys (i.e. as they are called before translation), e.g. for feedback in case of `KeyError`, or for silently suppressing non-permitted keys if one chooses to.

Technically `properties + translations` is the list of permitted keys, but conceptually I feel the list should only contain `:from` value for translated options. The latter is what I implemented. 

May be the `permitted_input_keys` could be made to accept a `with_translated_keys` option to provide implementation of both senses?

This patch is on top of my previous pull request (#128) that turned `Trash.translations` into a hash mapping original to translated keys.