Implement Flags parser and handle choose --everyone

- A generic Flags parser that could handle any `--flag` argument.
- `choose --everyone` overrides the Offline/DnD check.

youmubot-prelude/src/flags.rs

@@ -0,0 +1,38 @@
+use serenity::prelude::Args;
+use std::collections::HashSet as Set;
+
+/// Handle flags parsing.
+pub struct Flags(Set<String>);
+
+struct Flag(pub String);
+
+impl std::str::FromStr for Flag {
+    type Err = ();
+
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        if s.starts_with("--") {
+            Ok(Flag(s.trim_start_matches("--").to_owned()))
+        } else {
+            Err(())
+        }
+    }
+}
+
+impl Flags {
+    /// Parses the set of flags from a given `Args` structure.
+    pub fn collect_from(args: &mut Args) -> Flags {
+        let mut set = Set::new();
+        loop {
+            if let Some(Flag(s)) = args.find().ok() {
+                set.insert(s);
+            } else {
+                break Flags(set);
+            }
+        }
+    }
+
+    /// Checks whether `flag` exists in the flags set.
+    pub fn contains(&self, flag: impl AsRef<str>) -> bool {
+        self.0.contains(flag.as_ref())
+    }
+}