Add logSilent option to suppress logs

This is a simple modification to allow a new option `logSilent` to suppress log messages.

How to use:
```
var si = require('search-index')({ logSilent: true });
// si will not log anything from now on
```
Since the logger is global, there is no way to use different `logSilent` values when using multiple search-index instances (is this really supported anyway?). I strongly recommend modifying this lib to make it object-friendly, so that we can use multiple search indexes in the same process!