More closely mimic default SASL tty logger behavior: SASL progress
reports are not written to the tty/console.

Added a new environment config knob for the riak_err app,
console_error_type, to provide basic error level filtering
for the tty/console-mimicing behavior: 'all' (the default)
for all non-SASL-progress reports or 'error' for error-severity
non-SASL-progress reports.

Refactored the code so that it's possible to export a couple of
useful functions, format_event/3 and limited_fmt/4.  The refactoring
also made Eunit tests for the formatting much easier to write.

See the Edoc at the head of riak_err_app.erl for descriptions of
console_error_type and console_sasl_reports.