Added optional allowable_tags arguments to strip_tags() and fgetss() to

Added optional allowable_tags arguments to strip_tags() and fgetss() to
allow you to specify a string of tags that are not to be stripped

Could have used flex for this, and Andrew sent me a flex file to do this,
but I could do the same thing with only minor additions to the existing
state machine and the resulting code is much smaller and tighter.