Add before/after framework command hooks

These hooks will each be run prior to or after the command, and will
finish execution before executing the command.

These can be configured in a Framework via:

```rs
client.with_framework(|f| f
    .before(|_context, message, _command_name| {
        println!("Got command '{}'", command_name);
    })
    .after(|_context, _message, command_name| {
        println!("Finished command '{}'", command_name);
    }));
```

This does introduce a backwards compatibility break, by requiring
commands' Context/Message to be borrowed

Upgrade path:

If not using the `command!` macro, modify command signatures from:

```rs
fn some_command(context: Context, message: Message, args: Vec<String>)
```

to

```rs
fn some_command(context: &Context, message: &Message, args: Vec<String>)
```

1 files changed, 12 insertions, 8 deletions

diff --git a/examples/06_command_framework.rs b/examples/06_command_framework.rs
index af8781c..38f756d 100644
--- a/examples/06_command_framework.rs
+++ b/examples/06_command_framework.rs
@@ -12,10 +12,6 @@ fn main() {
         .expect("Expected a token in the environment");
     let mut client = Client::login_bot(&token);
 
-    client.on_message(|_context, message| {
-        println!("Received message: {:?}", message);
-    });
-
     client.on_ready(|_context, ready| {
         println!("{} is connected!", ready.user.name);
     });
@@ -28,9 +24,17 @@ fn main() {
     // "~some complex command"
     client.with_framework(|f| f
         .configure(|c| c
-            .on_mention(true)
             .allow_whitespace(true)
+            .on_mention(true)
             .prefix("~"))
+        .before(|_context, message, command_name| {
+            println!("Got command '{}' by user '{}'",
+                     command_name,
+                     message.author.name);
+        })
+        .after(|_context, _message, command_name| {
+            println!("Processed command '{}'", command_name)
+        })
         .on("ping", ping_command)
         .set_check("ping", owner_check) // Ensure only the owner can run this
         .on("emoji cat", cat_command)
@@ -47,11 +51,11 @@ command!(cat_command(context, _msg, _arg) {
     let _ = context.say(":cat:");
 });
 
-fn dog_command(context: Context, _msg: Message, _args: Vec<String>) {
+fn dog_command(context: &Context, _msg: &Message, _args: Vec<String>) {
     let _ = context.say(":dog:");
 }
 
-fn ping_command(_context: Context, message: Message, _args: Vec<String>) {
+fn ping_command(_context: &Context, message: &Message, _args: Vec<String>) {
     let _ = message.reply("Pong!");
 }
 
@@ -60,7 +64,7 @@ fn owner_check(_context: &Context, message: &Message) -> bool {
     message.author.id == 7
 }
 
-fn some_complex_command(context: Context, _msg: Message, args: Vec<String>) {
+fn some_complex_command(context: &Context, _msg: &Message, args: Vec<String>) {
     let _ = context.say(&format!("Arguments: {:?}", args));
 }