revert concat $warn/$warn_message param split + add deprecation warnings

Partially reverting the $warn/$warn_message param split from eaf8407 as this
change was unnecessarily API breaking.  Instead, we are adding string/bool type
validating to the $warn parameter and deprecation warnings for the usage of
stringified boolean values (eg, 'true', 'on', etc.). In a future major release,
the logic can be simplified to treating all string values as a warning message.
Merge pull request #124 from jhoblitt/revert_warn_message

revert concat $warn/$warn_message param split + add deprecation warnings