This commit adds a `Loader` type, which can be used to load multiple
source strings. This was done to support the work on modules, but
coincidentally enabled consolidating errors, since now `Config::run`
can take a `&Loader`, and in the event of an error, return and `Error`
that borrows from loaded strings. Multiple error types have been
consolidated, and a bunch of ad-hoc error printing was removed.
The `--choose` subcommand runs a chooser to select a recipe to run. The
chooser should read lines containing recipe names from standard input,
and write one of those names to standard output.
The chooser defaults to `fzf`, a popular fuzzy finder, but can be
overridden by setting $JUST_CHOOSER or passing `--chooser <CHOOSER>`.
