Add lang attribute to src code block to support Github syntax coloring
Merge branch 'lang-attribute-for-syntax-highlight'
Update test examples
Allow explicit enable or disable of include file feature

Include file feature disabled by default since
it would be dangerous in some environments

It will be activated by one of the following:

- setting an ORG_RUBY_ENABLE_INCLUDE_FILES env variable to 'true'
- setting an ORG_RUBY_INCLUDE_ROOT env variable with the root path
- explicitly enabling it by passing it as an option:
  e.g. Orgmode::Parser.new(org_text, { :allow_include_files => true })

IMPORTANT: To avoid the feature altogether, it can be _explicitly disabled_ as follows:
  e.g. Orgmode::Parser.new(org_text, { :allow_include_files => false })
Merge branch 'allow-explicit-disable-include-file'
Update markdown output test example
Bump version to 0.9.0
Update changelog