Unify flag descriptions to generate both docs and code

We want to specify all of GHC's flags in one place, and from there

• Generate the flag reference section of the documentation

• Generate the code to parse the command-line

• Allow flags to be deprecated, generating a warning

• Have an option to list all the flags that GHC supports (see #1226 (closed))

  for use in command-line completion

• Generate the man page

• Completion in GHCi

This ticket replaces part of #1226 (closed).