Add a script option to cabal init

[philderbeast]

Running cabal init is convenient. Could we also add an option to generate a single-file cabal script? This will save having to remember the exact comment syntax.

[ulysses4ever]

Mildly against because the weight of the implementation doesn't seem to justify the profit...

[Bodigrim]

I was thinking just today about having it vice versa: it would be nice to ask cabal init to elaborate a standalone Cabal script file into a package. I often start prototyping in a single file with {- cabal: ... -}, but at some point I need to convert it into a full-fledged package.