Transfer logging,trace levels from old to new global loggers.
Track whether global logging levels were set explicitly by the user or not.
set global logging levels from project settings if not already explicitly set
use global logging levels as the default for task logging levels
provide Logger via Keys.sLog for use by settings during project initialization.  Ref #806.
Deprecate -, --, and --- in favor of onFailure, sbtClearOnFailure, and resumeFromFailure.
early commands: anything prefixed with -- gets moved to the front of the command sequence on startup
clean up show command description to use string interpolation
Fix API comment for State.{+,++}
error,warn,info,debug commands to set log level, useful in conjunction with early combinator --.  Fixes #806.