Add cli option for voluntary exits (#1781)

## Issue Addressed

Resolve #1652 

## Proposed Changes

Adds a cli option for voluntary exits. The flow is similar to prysm's where after entering the password for the validator keystore (or load password from `secrets` if present) the user is given multiple warnings about the operation being irreversible, then redirected to the docs webpage(not added yet) which explains what a voluntary exit is and the consequences of exiting and then prompted to enter a phrase from the docs webpage as a final confirmation. 

Example usage
```
$ lighthouse --testnet zinken account validator exit --validator <validator-pubkey> --beacon-node http://localhost:5052

Running account manager for zinken testnet                                                                                                          
validator-dir path: "..."

Enter the keystore password:  for validator in ...

Password is correct

Publishing a voluntary exit for validator: ...              
WARNING: This is an irreversible operation                                                                                                                    
WARNING: Withdrawing staked eth will not be possible until Eth1/Eth2 merge Please visit [website] to make sure you understand the implications of a voluntary exit.            
                                                                                                                                             
Enter the phrase from the above URL to confirm the voluntary exit:
Exit my validator
Published voluntary exit for validator ...
```

## Additional info

Not sure if we should have batch exits (`--validator all`) option for exiting all the validators in the `validators` directory. I'm slightly leaning towards having only single exits but don't have a strong preference.

Cargo.lock

@@ -12,6 +12,7 @@ dependencies = [
  "directory",
  "dirs 3.0.1",
  "environment",
+ "eth2",
  "eth2_keystore",
  "eth2_ssz",
  "eth2_ssz_derive",
@@ -23,10 +24,13 @@ dependencies = [
  "libc",
  "rand 0.7.3",
  "rayon",
+ "safe_arith",
  "slashing_protection",
  "slog",
  "slog-async",
  "slog-term",
+ "slot_clock",
+ "tempfile",
  "tokio 0.2.22",
  "types",
  "validator_dir",