prepend option starts filename with '.'

[kidplug]

The default date pattern starts with a '.' - i'm guessing for easy appending.

  this.datePattern = options.datePattern ? options.datePattern : '.yyyy-MM-dd';

When the prepend option is used, the filename winds up starting with .

in _getFilename, add the leading or trailing '.' there.

eg:

if (this.prepend) {
    return formattedDate + '.' + this._basename;
  }

  return this._basename + '.' + formattedDate;

or:

if (this.prepend) {
    return [formattedDate,this._basename].join('.');
  }

  return [this._basename,formattedDate].join('.');

[mattberther]

Resolved by #42 and published to npm as v1.1.2.