Feature tweaks and ehancements

context.php:
    Allow accessing first, second or third elements from an array
    Access does not depend on array keys being [0,1,2,3...]
filters.php:
    Access second or third index from items
    Added json filter
    Limiting words could sometimes cut an html tag in half
tags.php:
    Allow start index in {% for %} tag
Removed second and third contexts
Changed access to last context so that it doesn't depend on sequential numerical indexes
elseif support
Merge remote-tracking branch 'upstream/master'
Merge remote-tracking branch 'IJMacD/master'
Merge remote-tracking branch 'ludoo/master'

Conflicts:
	h2o/tags.php