std: implement ParseFragment for Option

Closes #17

src/std.rs

@@ -3,6 +3,4 @@
 
 mod bool;
 mod number;
-
-pub use self::bool::*;
-pub use number::*;
+mod option;

src/std/option.rs

@@ -0,0 +1,37 @@
+use crate::core::{EString, ParseFragment};
+
+impl<T> ParseFragment for Option<T>
+where
+    T: ParseFragment,
+{
+    fn parse_frag(es: EString) -> crate::Result<Self> {
+        if es.is_empty() {
+            Ok(None)
+        } else {
+            es.parse().map(Some)
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn should_parse_empty_string_as_none() {
+        let estr = EString::from("");
+        match estr.parse::<Option<i32>>() {
+            Ok(res) => assert_eq!(res, None),
+            _ => unreachable!(),
+        }
+    }
+
+    #[test]
+    fn should_parse_number_as_some() {
+        let estr = EString::from("99");
+        match estr.parse::<Option<i32>>() {
+            Ok(res) => assert_eq!(res, Some(99)),
+            _ => unreachable!(),
+        }
+    }
+}