cli: drop single-letter aliases, keep the familiar three-letter ones where useful
expose the accounts command in Hledger.Cli
addons: simplify and firm up add-on command parsing

Drop the special case where we hide an add-on's source version if a
compiled version is also present. Better to be simple and explicit.

Improve robustness of command parsing, eg "hledger addon.hs"
will now work even though the command is listed as "addon".

And ignore any add-ons which would shadow a built-in command
(or any of the official command aliases displayed in the command list,
like "bal" and "reg"). Built-ins may not be replaced by an add-on.