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)
+        )
 }