Allow for nil prefixes in DMs

Fixes #339

src/framework/standard/command.rs

@@ -1,6 +1,9 @@
 use client::Context;
 use model::{
-    channel::Message,
+    channel::{
+        Message,
+        Channel,
+    },
     Permissions
 };
 use std::{
@@ -353,6 +356,20 @@ pub fn positions(ctx: &mut Context, msg: &Message, conf: &Configuration) -> Opti
             }
         }
 
+        #[cfg(feature = "cache")]
+        {
+            let private = match msg.channel() {
+                Some(Channel::Private(_)) => true,
+                _ => false,
+            };
+
+            // If the above do not fill `positions`, then that means no kind of prefix was present.
+            // Check if a no-prefix-execution is applicable.
+            if conf.no_prefix && private && positions.is_empty() {
+                positions.push(0);
+            }
+        }
+
         if positions.is_empty() {
             return None;
         }

src/framework/standard/configuration.rs

@@ -53,6 +53,7 @@ pub struct Configuration {
     #[doc(hidden)] pub on_mention: Option<Vec<String>>,
     #[doc(hidden)] pub owners: HashSet<UserId>,
     #[doc(hidden)] pub prefixes: Vec<String>,
+    #[doc(hidden)] pub no_prefix: bool,
     #[doc(hidden)] pub delimiters: Vec<String>,
     #[doc(hidden)] pub case_insensitive: bool,
 }
@@ -398,6 +399,16 @@ impl Configuration {
         self
     }
 
+    /// Sets whether command execution can done without a prefix. Works only in private channels.
+    ///
+    /// # Note
+    /// Needs the `cache` feature to be enabled. Otherwise this does nothing.
+    pub fn no_prefix(mut self, b: bool) -> Self {
+        self.no_prefix = b;
+
+        self
+    }
+
     /// Sets a delimiter to be used when splitting the content after a command.
     ///
     /// # Examples
@@ -466,6 +477,7 @@ impl Default for Configuration {
     /// - **depth** to `5`
     /// - **on_mention** to `false` (basically)
     /// - **prefix** to `None`
+    /// - **no_prefix** to `false`
     /// - **delimiters** to vec![" "]
     /// - **case_insensitive** to `false`
     fn default() -> Configuration {
@@ -475,6 +487,7 @@ impl Default for Configuration {
             dynamic_prefix: None,
             allow_whitespace: false,
             prefixes: vec![],
+            no_prefix: false,
             ignore_bots: true,
             owners: HashSet::default(),
             blocked_users: HashSet::default(),