feat: add slug, help, and version opts to binary

Adds some really basic cmdline argument parsing to the binary,
supporting help text, version display, and printing of a slug rather
than a full CUID.

src/bin.rs

@@ -1,13 +1,63 @@
-use cuid::cuid;
-use std::process::exit;
+use cuid::{cuid, slug};
+use std::{
+    env::{self, Args},
+    process::exit,
+};
 
 /// Generate a new CUID and print it to stdout
 pub fn main() {
-    match cuid() {
+    let args: CuidArgs = env::args().into();
+
+    let res = if args.slug { slug() } else { cuid() };
+
+    match res {
         Ok(id) => println!("{}", id),
         Err(err) => {
             eprintln!("{:?}", err);
             exit(1)
         }
     }
 }
+
+const HELP: &'static str = r#"Usage: cuid [OPTION]...
+Generate and print a CUID.
+
+Options:
+  --slug         generate a slug instead of a full CUID
+  -h, --help     display this help and exit
+  -v, --version  display version information and exit"#;
+
+const VERSION: &'static str = env!("CARGO_PKG_VERSION");
+
+/// Commandline arguments for the CUID binary
+#[derive(Debug)]
+struct CuidArgs {
+    /// Whether to produce a slug instead of a CUID
+    slug: bool,
+}
+impl From<Args> for CuidArgs {
+    fn from(args: Args) -> Self {
+        let mut slug = false;
+
+        // The first argument should be the binary name. Skip it.
+        args.skip(1).for_each(|arg| match arg.as_str() {
+            "-h" | "--help" => {
+                println!("{}", HELP);
+                exit(0);
+            }
+            "-v" | "--version" => {
+                println!("{}", VERSION);
+                exit(0);
+            }
+            "--slug" => slug = true,
+            _ => {
+                println!("error: unrecognized argument {}", arg);
+                println!("");
+                println!("{}", HELP);
+                exit(1);
+            }
+        });
+
+        CuidArgs { slug }
+    }
+}