Set Graffiti via CLI (#1320)

## Issue Addressed

Closes #1319 

## Proposed Changes

This issue:
1. Allows users to edit their Graffiti via the cli option `--graffiti`. If the graffiti is too long, lighthouse will not start and throw an error message. Otherwise, it will set the Graffiti to be the one provided by the user, right-padded with 0s.
2. Create a new `Graffiti` type and unify the code around it. With this type, everything is enforced at compile-time, and the code can be (I think...) panic-free! :)

## Additional info

Currently, only `&str` are supported, as this is the returned type by `.arg("graffiti")`.
Since this is user-input, I tried being as careful as I could. This is also why I created the `Graffiti` type, to make sure I could check as much as possible at compile time.

beacon_node/src/cli.rs

@@ -1,5 +1,11 @@
 use clap::{App, Arg};
 
+// Default text included in blocks.
+// Must be 32-bytes or will not build.
+//
+//                              |-------must be this long------|
+const DEFAULT_GRAFFITI: &str = "sigp/lighthouse-0.1.2-prerelease";
+
 pub fn cli_app<'a, 'b>() -> App<'a, 'b> {
     App::new("beacon_node")
         .visible_aliases(&["b", "bn", "beacon"])
@@ -227,4 +233,16 @@ pub fn cli_app<'a, 'b>() -> App<'a, 'b> {
                 .long("purge-db")
                 .help("If present, the chain database will be deleted. Use with caution.")
         )
+
+        /*
+         * Misc.
+         */
+        .arg(
+            Arg::with_name("graffiti")
+                .long("graffiti")
+                .help("Specify your custom graffiti to be included in blocks.")
+                .value_name("GRAFFITI")
+                .default_value(DEFAULT_GRAFFITI)
+                .takes_value(true)
+        )
 }

beacon_node/src/config.rs

@@ -10,7 +10,7 @@ use std::fs;
 use std::net::{IpAddr, Ipv4Addr, ToSocketAddrs};
 use std::net::{TcpListener, UdpSocket};
 use std::path::PathBuf;
-use types::{ChainSpec, EthSpec};
+use types::{ChainSpec, EthSpec, GRAFFITI_BYTES_LEN};
 
 pub const BEACON_NODE_DIR: &str = "beacon";
 pub const NETWORK_DIR: &str = "network";
@@ -343,6 +343,22 @@ pub fn get_config<E: EthSpec>(
         client_config.genesis = ClientGenesis::DepositContract;
     }
 
+    if let Some(graffiti) = cli_args.value_of("graffiti") {
+        let graffiti_bytes = graffiti.as_bytes();
+        if graffiti_bytes.len() > GRAFFITI_BYTES_LEN {
+            return Err(format!(
+                "Your graffiti is too long! {} bytes maximum!",
+                GRAFFITI_BYTES_LEN
+            ));
+        } else {
+            // `client_config.graffiti` is initialized by default to be all 0.
+            // We simply copy the bytes from `graffiti_bytes` in there.
+            //
+            // Panic-free because `graffiti_bytes.len()` <= `GRAFFITI_BYTES_LEN`.
+            client_config.graffiti[..graffiti_bytes.len()].copy_from_slice(graffiti_bytes);
+        }
+    }
+
     Ok(client_config)
 }