TableNodeFormatter#visit raises MalFormedTableError when a row does not have the sufficient number of columns
substituted the parameter "verbose_mode" in PlainTextFormat#initialize "with options"
substituted the parameter of PlainTextFormat.create from "verbose_mode" to "options"
TableNodeFormatter#visit raises MalFormedTableError only if @option.strict_mode is true
changed the error message of MalFormedTableError
a tidy-up
added a task, generate_visitor to Rakefile
correct a typo
removed unused subclasses of PlainTextFormat
removed commented out subclasses of PlainTextFormat
implemented DelNodeFormatter#visit