Join variables by their delimiter in quoted expansion

This switches quoted expansion like "$foo" to use foo's delimiter instead of
space. The delimiter is space for normal variables and colonf or path variables.
Expansions like "$PATH" will now expand using ':'.

tests/expansion.out

@@ -73,3 +73,7 @@ All digits: 0
 
 ####################
 # Test tilde expansion
+
+####################
+# Test delimiters
+one two three one:two:three