parser: options.pos, settings, named, CodeBlock
parser: options.pos, settings, named, CodeBlock

- updated util 'append' to work with 'options.pos' and
'options.appendSource'
- added 'settings' field to grammer ast
- added '@set' directive that adds settings
- updated util 'extractList' to ignore empty elements
- removed 'named' type, now its 'display' field in 'rule' type
- 'CodeBlock' can now optionally return a string used in generators to
set a return type

erispa/pegx