Merge branch 'feature/comments' into feature/jastadd-extension

src/main/jastadd/RelAst.relast

src/main/jastadd/backend/AbstractGrammar.jadd

src/main/jastadd/parser/RelAst.parser

src/main/jastadd/scanner/Header.flex

src/main/jastadd/scanner/Keywords.flex

src/main/jastadd/scanner/Macros.flex

src/main/jastadd/scanner/RulesPostamble.flex

src/main/jastadd/scanner/RulesPreamble.flex

src/main/jastadd/scanner/Symbols.flex