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));
```

1 files changed, 1 insertions, 1 deletions

diff --git a/src/client/dispatch.rs b/src/client/dispatch.rs
index 985f03e..6295bc9 100644
--- a/src/client/dispatch.rs
+++ b/src/client/dispatch.rs
@@ -1,5 +1,5 @@
 use std::sync::{Arc, Mutex};
-use std::{mem, thread};
+use std::thread;
 use super::event_store::EventStore;
 use super::login_type::LoginType;
 use super::{STATE, Connection, Context};