stacktraces: `clj-stacktrace`->`io.aviso.exception` for stacktrace printing
nb - major refactor (see commit msg for details)

CHANGES
 * DEPRECATED: `red`, `green`, `blue` -> use `color-str` instead.
 * Logging level is now located in `level-atom` rather than `config`.
   This is in prep for new config types. Old in-config levels will be
   respected (i.e. change is backwards compatible).
 * DEPRECATED: config `prefix-fn` has been replaced by the more flexible
   `fmt-output-fn`. Change is backwards compatible.
 * Built-in appenders have been simplified using the new `default-output`
   appender arg.
 * Per-appender `:prefix` option dropped - was unnecessary. If an appender
   wants custom output formatting, it can do so w/o using an in-config
   formatter.

FIXES
 * Messages are now generated _after_ middleware has been applied, allowing
   better filtering performance and more intuitive behaviour (e.g. changes
   to args in middleware will now automatically percolate to message content).
Update CHANGELOG (+adopting `cider` style)