Add filter to args for purchase link

Currently you can only modify the default args. This doesn't give you access to the actual download if used in a shortcode (as it's the current post ID).
Merge pull request #2070 from elliott-stocks/patch-12

Add filter to args for purchase link
Compress admin-scripts
Merge branch 'master' of github.com:easydigitaldownloads/Easy-Digital-Downloads
Increment API version and set discount to array if none
Merge branch 'master' of github.com:easydigitaldownloads/Easy-Digital-Downloads