Experimental: add middleware support, refactor appender decoration

* Appenders are now decorated via 2 separate mechanisms: a per-appender
  decorator (as before), and a new per-juxt (i.e. combined) decorator.
  The per-juxt wrapping provides a performance-conscious hook for
  higher-level facilities like the new middleware feature.

* Moved compile-time config wrapping from per-appender to per-juxt,
  improving performance.

* Fixed appender wrapper ordering.
Bump version (1.4.0)