Set `meta.mainProgram`

[roberth]

When the cabal file has a single executable, cabal2nix can generate a meta.mainProgram attribute, so that the command name can be used by generic code in Nixpkgs.

Example use cases:

• pkg.exe shortcut, e.g. ${pandoc.exe} --foo Add exe attribute to packages nixpkgs#158461
• nix run needs this info to run the right command when it doesn't match the package name

[sternenseemann]

First step is done and ready for review: NixOS/distribution-nixpkgs#20