Make builders mutably borrowed

Change the builders so that they are now mutably borrowed, accepting
`&mut self` instead of `self`. Their methods now return `()` instead of
`Self`.

Upgrade path:

Change code such as the following:

```rust
channel.send_message(|m| m
    .embed(|e| e
        .description("test")
        .title("title")));
```

to the following style:

```rust
channel.send_message(|mut m| {
    m.embed(|mut e| {
        e.description("test");
        e.title("title");

        e
    });

    m
});
```

Closes #159.

1 files changed, 6 insertions, 8 deletions

diff --git a/src/builder/edit_message.rs b/src/builder/edit_message.rs
index 29da46a..7c15e2b 100644
--- a/src/builder/edit_message.rs
+++ b/src/builder/edit_message.rs
@@ -14,8 +14,11 @@ use utils::{self, VecMap};
 /// #
 /// # let mut message = ChannelId(7).message(MessageId(8)).unwrap();
 /// #
+/// let _ = message.edit(|mut m| {
+///     m.content("hello");
 ///
-/// let _ = message.edit(|m| m.content("hello"));
+///     m
+/// });
 /// ```
 ///
 /// [`Message`]: ../model/channel/struct.Message.html
@@ -26,20 +29,15 @@ impl EditMessage {
     /// Set the content of the message.
     ///
     /// **Note**: Message contents must be under 2000 unicode code points.
-    pub fn content<D: Display>(mut self, content: D) -> Self {
+    pub fn content<D: Display>(&mut self, content: D) {
         self.0.insert("content", Value::String(content.to_string()));
-
-        self
     }
 
     /// Set an embed for the message.
-    pub fn embed<F>(mut self, f: F) -> Self
-        where F: FnOnce(CreateEmbed) -> CreateEmbed {
+    pub fn embed<F: FnOnce(CreateEmbed) -> CreateEmbed>(&mut self, f: F) {
         let map = utils::vecmap_to_json_map(f(CreateEmbed::default()).0);
         let embed = Value::Object(map);
 
         self.0.insert("embed", embed);
-
-        self
     }
 }