Added filter to allow modifying the variable prices

For simple downloads, the download price can be altered by adding a `edd_get_download_price` hook. The new hook will allow to do the same with variable prices, which, at the moment, cannot be modified if needed, because they are used straight away, in their raw form, as soon as they are loaded.
Added download_id to edd_get_variable_prices filter

As requested by @pippinsplugins.
Merge pull request #2254 from daigo75/master

Added filter to allow modifying the variable prices