Move variable filtering into the init action

Prevents race conditions where it’s impossible to filter these values
if the Zoninator code is included before the filter is registered.

This gives all plugins and themes the chance to register their filters
at any point (before `init 99`) and have them properly executed.
Merge pull request #20 from nickdaugherty/delay-filter-execution

Move variable filtering into the init action
Merge remote-tracking branch 'upstream/master'

Conflicts:
	zoninator.php