Disable attribute with string parameter should write string to function.json

[lindydonna]

Do the following:

• Match against a regex of alphanumeric strings. Anything else is a build error.
• If %setting% is used, build error should say '%' expressions are not supported for 'Disable'. Use 'Disable("setting") instead of 'Disable("%setting%")'
• [Disable("setting")] writes "disabled": "setting" to function.json
• Build error if they use constructor DisableAttribute(Type providerType). Error: the constructor 'DisableAttribute(Type)' is not supported.

I'll update the doccomment for DisableAttribute so that it's clear that the extra features work only in WebJobs.

[MikeStall]

The driving factor here is that whatever the tooling emits cooperates with the existing function.json spec around disabled; and if it's not supported in function.json today, then the tooling emits a error.