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/client/src/config.rs

@@ -2,6 +2,7 @@ use network::NetworkConfig;
 use serde_derive::{Deserialize, Serialize};
 use std::fs;
 use std::path::PathBuf;
+use types::Graffiti;
 
 pub const DEFAULT_DATADIR: &str = ".lighthouse";
 
@@ -55,6 +56,8 @@ pub struct Config {
     pub sync_eth1_chain: bool,
     /// A list of hard-coded forks that will be disabled.
     pub disabled_forks: Vec<String>,
+    /// Graffiti to be inserted everytime we create a block.
+    pub graffiti: Graffiti,
     #[serde(skip)]
     /// The `genesis` field is not serialized or deserialized by `serde` to ensure it is defined
     /// via the CLI at runtime, instead of from a configuration file saved to disk.
@@ -84,6 +87,7 @@ impl Default for Config {
             sync_eth1_chain: false,
             eth1: <_>::default(),
             disabled_forks: Vec::new(),
+            graffiti: Graffiti::default(),
         }
     }
 }