Disable rendering of tag based on register (#1162)

* Disable rendering of tag based on register

* Improvements to disable tag

* Resolve disbale tag tests

* Test disable_tags register

* disabled_tags is now always avaiable

* Allow multiple tags to be disabled at once

* Move disabled check to block_body

* Code improvements

* Remove redundant nil check

* Improve disabled tag error output

* Improve disable tag API

* Code improvements

* Switch disabled? to not mutate output

* Fix array handling shortcut in disable_tags

lib/liquid.rb

@@ -78,8 +78,10 @@ require 'liquid/tokenizer'
 require 'liquid/parse_context'
 require 'liquid/partial_cache'
 require 'liquid/usage'
+require 'liquid/register'
 require 'liquid/static_registers'
 
 # Load all the tags of the standard library
 #
 Dir["#{__dir__}/liquid/tags/*.rb"].each { |f| require f }
+Dir["#{__dir__}/liquid/registers/*.rb"].each { |f| require f }