add quoted labels

lib/DhallishGrammar.treetop

@@ -427,7 +427,8 @@ grammar DhallishGrammar
 	end
 
 	rule label
-		("_" / [a-zA-Z]) ("_" / "-" / "/" / [a-zA-Z0-9])* { def to_node(ctx) Dhallish::Ast::VariableNode.new text_value end }
+		("_" / [a-zA-Z]) ("_" / "-" / "/" / [a-zA-Z0-9])* { def to_node(ctx) Dhallish::Ast::VariableNode.new text_value end } /
+		"`" lb:(([a-zA-Z0-9] / "-" / "/" / "_" / ":" / "." / "$")+) "`" { def to_node(ctx) Dhallish::Ast::VariableNode.new lb.text_value end }
 	end
 
 	rule let_expression
@@ -481,7 +482,7 @@ grammar DhallishGrammar
 
 	rule import_source
 		# can be an environment variable, url or path
-		(![\s] .)*
+		(!([\s] / "\\" / "<" / ">" / "?" / "," / "[" / "]" / "{" / "}" / "#" / "(" / ")") .)*
 	end
 
 end