Support symbol and string for the st+... functions to get variable

src/lib.rs

@@ -97,13 +97,16 @@ To access the values in lisp you can use the following functions:
 - `st+num` the value as a number, and
 - `st+has` true if value is present else false.
 
-You need to quote the symbol to pass to the functions (e.g. (st+num 'total).
+You need to quote the symbol to pass to the functions (e.g. (st+num
+'total) or (st+num "total").
 
 Else, you can just write the variables in braces like normal as well.
 
 there are two use cases.
-- Standalone use case: in the format of `=()` that'll be replaced with the results.
-- Inside the `{}` that can be used as alternative to a variable, or with a transformer.
+- Standalone use case: in the format of `=()` that'll be replaced with
+  the results.
+- Inside the `{}` that can be used as alternative to a variable, or
+  with a transformer.
 
 ```rust
 # use std::error::Error;
@@ -112,7 +115,7 @@ there are two use cases.
 # use string_template_plus::{Render, RenderOptions, Template};
 #
 # fn main() -> Result<(), Box<dyn Error>> {
-let templ = Template::parse_template("hello {nickname?name}. You've done =(/ (st+num 'task_done) (st+num 'task_total)) work. {=(- 1 (/ (st+num 'task_done) (st+num 'task_total))):calc(*100):f(1)}% remains.").unwrap();
+let templ = Template::parse_template("hello {nickname?name}. You've done =(/ (st+num 'task_done) (st+num 'task_total)) work. {=(- 1 (/ (st+num \"task_done\") (st+num 'task_total))):calc(*100):f(1)}% remains.").unwrap();
 let mut vars: HashMap<String, String> = HashMap::new();
 vars.insert("name".into(), "world".into());
 vars.insert("task_done".into(), "1".into());