Diagnostic output: list cowpath and prefix, etc

[apjanke]

Could be useful to have some diagnostic or debugging output, like in a cowsay -D invocation, to help diagnose issues like #49, where we're not sure what the effective cowpath and prefix discovery is.

In the output, I'd want something like:

• The effective cowpath, listing all the directories on it.
  • Maybe note which ones were baked in based on the prefix, and which came from the $COWPATH env var, and which came from cowpath.d entries (and where those cowpath.d entries came from).
  • And maybe whether $COWSAY_ONLY_COWPATH is in effect.
• The $prefix that cowsay discovered.
  • And whether cowsay thinks it's running in deployed or in-repo mode.
  • The effective $etc dir.
• The cowsay version, so the user doesn't have to run cowsay --version separately when producing diagnostic output to copy-paste into a bug report.

Maybe an expanded version of cowsay -l that indicates where each cow comes from and notes shadowed cows, to help diagnose "why isn't my customized cow working?" stuff.