add usePlaceholderId option
add usePlaceholderId option

mysqljs (others might too) requires query values to be escaped [via a question mark](https://github.com/mysqljs/mysql#performing-queries).
`usePlaceholderId` option is added In order to facilitate this behaviour
```javascript
let jsonSql = require('json-sql')({
  dialect: 'mysql',
  namedValues: false,
  valuesPrefix: '?',
  usePlaceholderId: false
});
```
2do2go/json-sql