Add new Gridicons

[frosty]

This PR adds a load of new Gridicons that have been added to the main repo since Gridicons-iOS was last updated:

• chat
• coupon
• crossCircle
• ellipsisCircle
• filter
• fullscreen
• fullscreenExit
• money
• offline
• pause
• product
• productDownloadable
• productExternal
• productVirtual
• readMore
• refund
• resize
• shipping
• userAdd

Because the code is auto-generated, the code for all of the Gridicons will have been updated.

To test:

• Run the Gridicons demo and look through all the icons to check everything looks okay.
• Reference this branch from WPiOS's Podfile, and check everything looks okay in the main app.

Needs review: @jleandroperez