Add 'apply_filters' to 'EDD_Download' class 'support_buy_now()' function return value

[maaprote]

Since EDD v3.2.2, in some custom payment processors plugins like FastSpring by ProtheusPay (https://proteuspay.com/), the 'Buy Now' functionality stop to work with recurring payments. If that's going to be the default behavior (don't work with recurring payments), the ideal would be at least provide a hook to be able to change that behavior with custom code.

Fixes #

Proposed Changes:

1. Encapsulate the logic to check whether buy now is supported or not in a new 'is_buy_now_supported' function to be able to apply the filter.
2. Add the filter 'edd_download_supports_buy_now' in the return value from function 'supports_buy_now'