Add support for additional Connection parameters

Would this do the trick? I was a bit unsure about having `options` as a second level within the main options array, but passing strings like 'port' and constant ints like MYSQLI_OPT_CONNECT_TIMEOUT in the same way did feel a bit weird. I suppose the `options` array could be dropped if a `ConnectionInterface` were added, and just let the end-user worry about that. Happy to make any fixes or style changes you'd suggest.

As discussed: 

* add `public function setParam()` that accepts a pair
* add `public function setParams()` that accepts an associative array
* add `public function getParams()` that returns an associative array
* put the @deprecated tag on `setConnectionParams()` and `getConnectionParams()`, with date and suggestion to use `setParams()` and `getParams()`
* replace instances of `setConnectionParams()` and `getConnectionParams()`
* update README

The parameters array should look like:

```php
[
   'host' => $host,
   'port' => $port,
   'options' => [
      MYSQLI_OPT_CONNECT_TIMEOUT => 2
   ]
]
```