Detailed validator monitoring (#2151)

## Issue Addressed

- Resolves #2064

## Proposed Changes

Adds a `ValidatorMonitor` struct which provides additional logging and Grafana metrics for specific validators.

Use `lighthouse bn --validator-monitor` to automatically enable monitoring for any validator that hits the [subnet subscription](https://ethereum.github.io/eth2.0-APIs/#/Validator/prepareBeaconCommitteeSubnet) HTTP API endpoint.

Also, use `lighthouse bn --validator-monitor-pubkeys` to supply a list of validators which will always be monitored.

See the new docs included in this PR for more info.

## TODO

- [x] Track validator balance, `slashed` status, etc.
- [x] ~~Register slashings in current epoch, not offense epoch~~
- [ ] Publish Grafana dashboard, update TODO link in docs
- [x] ~~#2130 is merged into this branch, resolve that~~

beacon_node/beacon_chain/src/attestation_verification.rs

@@ -778,6 +778,11 @@ impl<T: BeaconChainTypes> VerifiedUnaggregatedAttestation<T> {
         &self.attestation
     }
 
+    /// Returns the wrapped `indexed_attestation`.
+    pub fn indexed_attestation(&self) -> &IndexedAttestation<T::EthSpec> {
+        &self.indexed_attestation
+    }
+
     /// Returns a mutable reference to the underlying attestation.
     ///
     /// Only use during testing since modifying the `IndexedAttestation` can cause the attestation