Add lookup_mut method for mutable access

Mutable access may sometimes be desired in order to change values
in the toml table. This can be used for dynamic configurations which
will be easy to modify and store.

lookup_mut requires a recursive method due to the borrow checker
not allowing to have more than one mutable reference in the same
scope.

src/lib.rs

@@ -44,6 +44,7 @@
 
 use std::collections::BTreeMap;
 use std::str::FromStr;
+use std::str::Split;
 
 pub use parser::{Parser, ParserError};
 
@@ -206,6 +207,67 @@ impl Value {
 
         Some(cur_value)
     }
+
+    fn lookup_mut_recurse<'a>(&'a mut self, matches: &mut Split<'a, char>) -> Option<&'a mut Value> {
+        if let Some(key) = matches.next() {
+            match *self {
+                Value::Table(ref mut hm) => {
+                    match hm.get_mut(key) {
+                      Some(v) => return v.lookup_mut_recurse(matches),
+                      None => return None,
+                  }
+              },
+              Value::Array(ref mut v) => {
+                  match key.parse::<usize>().ok() {
+                      Some(idx) if idx < v.len()
+                          => return (&mut v[idx]).lookup_mut_recurse(matches),
+                        _ => return None,
+                    }
+                },
+            _ => return None
+            }
+        }
+        Some(self)
+    }
+
+    /// Lookups for mutable value at specified path.
+    ///
+    /// Uses '.' as a path separator.
+    ///
+    /// Note: arrays have zero-based indexes.
+    ///
+    /// Note: empty path returns self.
+    ///
+    /// ```
+    /// # #![allow(unstable)]
+    /// let toml = r#"
+    ///      [test]
+    ///      foo = "bar"
+    ///
+    ///      [[values]]
+    ///      foo = "baz"
+    ///
+    ///      [[values]]
+    ///      foo = "qux"
+    /// "#;
+    /// let mut value: toml::Value = toml.parse().unwrap();
+    /// {
+    ///    let string = value.lookup_mut("test.foo").unwrap();
+    ///    assert_eq!(string, &mut toml::Value::String(String::from("bar")));
+    ///    *string = toml::Value::String(String::from("foo"));
+    /// }
+    /// let result = value.lookup_mut("test.foo").unwrap();
+    /// assert_eq!(result.as_str().unwrap(), "foo");
+    /// ```
+    pub fn lookup_mut<'a>(&'a mut self, path: &'a str) -> Option<&'a mut Value> {
+      if path.len() == 0 {
+          return Some(self)
+      }
+
+      let mut matches = path.split('.');
+      self.lookup_mut_recurse(&mut matches)
+  }
+
 }
 
 impl FromStr for Value {