Add a message builder for Context::send_message

Add a message builder to `send_message`. Often only one field - i.e.
`content` - needs to be specified, and the rest can be ignored.

This is a preliminary patch to add rich embed support to messages.

This message builder is used via:

```rust
// assuming in a context with a `channel_id` bound
context.send_message(channel_id, |m| m
    .content("TTS ping!")
    .tts(true));
```

2 files changed, 73 insertions, 0 deletions

diff --git a/src/utils/builder/create_message.rs b/src/utils/builder/create_message.rs
new file mode 100644
index 0000000..dc601a4
--- /dev/null
+++ b/src/utils/builder/create_message.rs
@@ -0,0 +1,71 @@
+use serde_json::Value;
+use std::collections::BTreeMap;
+use std::default::Default;
+
+/// A builder to specify the contents of an [`http::create_message`] request,
+/// primarily meant for use through [`Context::send_message`].
+///
+/// `content` is the only required field.
+///
+/// Note that if you only need to send the content of a message, without
+/// specifying other fields, then [`Context::say`] may be a more preferable
+/// option.
+///
+/// # Examples
+///
+/// Sending a message with a content of `test` and applying text-to-speech:
+///
+/// ```rust,ignore
+/// // assuming you are in a context
+/// context.send_message(message.channel_id, |m|
+///     .content("test")
+///     .tts(true));
+/// ```
+///
+/// [`Context::say`]: ../../client/struct.Context.html#method.say
+/// [`Context::send_message`]: ../../client/struct.Context.html#method.send_message
+/// [`http::create_message`]: ../../client/http/fn.create_message.html
+pub struct CreateMessage(pub BTreeMap<String, Value>);
+
+impl CreateMessage {
+    /// Set the content of the message.
+    ///
+    /// **Note**: Message contents must be under 2000 unicode code points.
+    pub fn content(mut self, content: &str) -> Self {
+        self.0.insert("content".to_owned(), Value::String(content.to_owned()));
+
+        CreateMessage(self.0)
+    }
+
+    /// Set the nonce. This is used for validation of a sent message. You most
+    /// likely don't need to worry about this.
+    ///
+    /// Defaults to empty.
+    pub fn nonce(mut self, nonce: &str) -> Self {
+        self.0.insert("nonce".to_owned(), Value::String(nonce.to_owned()));
+
+        CreateMessage(self.0)
+    }
+
+    /// Set whether the message is text-to-speech.
+    ///
+    /// Think carefully before setting this to `true`.
+    ///
+    /// Defaults to `false`.
+    pub fn tts(mut self, tts: bool) -> Self {
+        self.0.insert("tts".to_owned(), Value::Bool(tts));
+
+        CreateMessage(self.0)
+    }
+}
+
+impl Default for CreateMessage {
+    /// Creates a map for sending a [`Message`], setting `tts` to `false` by
+    /// default.
+    fn default() -> CreateMessage {
+        let mut map = BTreeMap::default();
+        map.insert("tts".to_owned(), Value::Bool(false));
+
+        CreateMessage(map)
+    }
+}
diff --git a/src/utils/builder/mod.rs b/src/utils/builder/mod.rs
index cd830fa..91ad940 100644
--- a/src/utils/builder/mod.rs
+++ b/src/utils/builder/mod.rs
@@ -7,6 +7,7 @@
 
 mod create_embed;
 mod create_invite;
+mod create_message;
 mod edit_channel;
 mod edit_guild;
 mod edit_member;
@@ -17,6 +18,7 @@ mod get_messages;
 
 pub use self::create_embed::CreateEmbed;
 pub use self::create_invite::CreateInvite;
+pub use self::create_message::CreateMessage;
 pub use self::edit_channel::EditChannel;
 pub use self::edit_guild::EditGuild;
 pub use self::edit_member::EditMember;