Translators comment (fixes #127)

See #127.

Also, this will help with #131 future changes, since it upgrade `sources_parser` API to support such things as plural forms and msgctx (which still should be implemented in parser and compiler).

Please, note that I add inline comments to AST (they were just skipped by lexer before) and convert them to comment block tags. So, parse + unparse of `111{#222#}333` will produce `111{%comment%}222{%endcomment%}333`.

Examples are in `sources_parser` itself and in tests.