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, 7 insertions, 1 deletions

diff --git a/src/model/user.rs b/src/model/user.rs
index 1e90cb4..af09d4e 100644
--- a/src/model/user.rs
+++ b/src/model/user.rs
@@ -438,7 +438,13 @@ impl User {
     ///                 url,
     ///             );
     ///
-    ///             match msg.author.direct_message(|m| m.content(&help)) {
+    ///             let dm = msg.author.direct_message(|mut m| {
+    ///                 m.content(&help);
+    ///
+    ///                 m
+    ///             });
+    ///
+    ///             match dm {
     ///                 Ok(_) => {
     ///                     let _ = msg.react('👌');
     ///                 },