Proposal: Add `Message#files?`

[jockee]

As it is in both Nylas' and its users' best interest to minimize the number of requests, I propose adding a method #files? to Message in order to remove the need to hit the files endpoint in order to know whether the message actually has files attached to it.

[khamidou]

Thanks for the request. I've just fixed this in eddf0e6.

[jockee]

Thanks @khamidou!