vim-plugins: use lower case attr and pkg names

(According to the nixpkgs manual.)

Keep old attrnames around for backward compatibility.
vim-plugins: coffee-script: start version number with digit

Or else nix will see it as part of package name.
vim-plugins: use commit dates instead of commit ids as version numbers

Git commit ids shouldn't be used as version numbers in nixpkgs:
* they don't increase monotonically
* they don't always start with a digit, causing nix to not parse them as
  version numbers, and instead sees them as part of the package name,
  causing the package to (seemingly) conflict with itself upon upgrade.
vim-plugins: sort alphabetically