Add support for method calls in blocks

-- Add configuration variable for a module that contains helper methods
   that can be called from blocks
-- Parse block content and if it begins with "#!", it is considered an
   executable block. Parse it to find the method name and an optional
   JSON options hash to be passed to the method. If validated, and the
   method exists in the configured module, run it and return the content.
   Otherwise, just return the content from the database as usual.
-- Use ordinary textarea for admin tool to avoid automatic linting
   by wsywig editor.