Possibility to change the key after you have initialised the rate limiter
To reuse the same code for different endpoints it would be useful to change the key after you have initialised the rate limiter.

Current code:
```` javascript
const {RateLimiterPostgres} = require('rate-limiter-flexible');

const opts = {
  points: 6,
  duration: 1,
  storeClient: sequelize,
  tableName: 'RateLimit',
  keyPrefix: 'UsernameLookup'
};

const ready = (err) => {
  if (err) {
    console.error(err);
  } else {
    console.log('Created/Found table needed for rate limiting');
  }
};

const rateLimiter = new RateLimiterPostgres(opts, ready);

exports.rateLimiting = (req, res, next) => {
  rateLimiter.consume(req.ip, 1)
    .then((rateLimiterRes) => {
      next();
    })
    .catch(() => {
      res.status(429).send('Too Many Requests');
    });
};
````

It would be useful to change the key from within the rateLimiting function, so the same export can be used for different endpoints if required. 

Example: 
```` javascript
const {RateLimiterPostgres} = require('rate-limiter-flexible');

const opts = {
  points: 6,
  duration: 1,
  storeClient: sequelize,
  tableName: 'RateLimit',
};

const ready = (err) => {
  if (err) {
    console.error(err);
  } else {
    console.log('Created/Found table needed for rate limiting');
  }
};

const rateLimiter = new RateLimiterPostgres(opts, ready);

exports.rateLimiting = (req, res, next) => {
  if (req.path.indexOf('username') > -1 {
    rateLimiter.key = 'UsernameLookup';
  } else  if (req.path.indexOf('passwordReset') > -1 {
    rateLimiter.key = 'PasswordReset';
  }

  rateLimiter.consume(req.ip, 1)
    .then((rateLimiterRes) => {
      next();
    })
    .catch(() => {
      res.status(429).send('Too Many Requests');
    });
};