message: Add mnemonic strength (length)
diff: import { generateMnemonic } from 'bip39';
  +type Strength = 12 | 24;
  +
  /**
  * @name mnemonicGenerate
  * @summary Creates a valid mnemonic string using using [BIP39](https://github.com/bitcoin/bips/blob/master/bip-0039.mediawiki).
  @@ -16,6 +18,6 @@ import { generateMnemonic } from 'bip39';
  * const mnemonic = mnemonicGenerate(); // => string
  * ```
  */
  -export default function mnemonicGenerate (): string {
  - return generateMnemonic();
  +export default function mnemonicGenerate (strength?: Strength): string {
  + return generateMnemonic(strength);
  }
  