Add support for creating / editing appveyor.yml

[zspitz]

I recently wrote my first appveyor.yml, and it was a rather long and error-prone process until I could get AppVeyor to successfully build and deploy my project.

Some sort of UI for editing appveyor.yml, or even some sort of wizard to create it, would be very helpful.

I know that there is a website UI, but it doesn't distinguish between the settings that can be set only in the UI, and settings that can be set only in appveyor.yml.