The previous cleanup defined long help separately from the usage text
generated by cmdargs. This meant keeping flag descriptions synced
between the two, and also the short help was often too verbose and
longer than the long help.
Now, the non-usage bits of long help are defined as pre and postambles
within the cmdargs mode, letting cmdargs generate the long help
including all flags. We derive the short help from this by truncating
at the start of the hledger common flags.
Most of the bundled addons (all but hledger-budget) now use the
new scheme and have pretty reasonable -h and --help output.
We can do more to reduce boilerplate for addon authors.
