Several changes, see commit msg

* clean up all tests
* bugfix for zero value functions, and test
* removed expand function, put in snippets
* added doc strings to Symbol type
* added doc strings to symbol declarations
* implemented display for Args type
* wrote a help function
* wrote docstrings for all builtins and config vars

src/segment.rs

@@ -240,9 +240,9 @@ impl fmt::Display for Ctr {
             Ctr::Float(s) => write!(f, "{}", s),
             Ctr::Bool(s) => {
                 if *s {
-                    write!(f, "T")
+                    write!(f, "true")
                 } else {
-                    write!(f, "F")
+                    write!(f, "false")
                 }
             }
             Ctr::Seg(s) => write!(f, "{}", s),