Allow Customized Line-Separator Character

We use the FFmpeg utility as part of our transcoding pipeline; it outputs many useful metrics like decode frame-rate, bitrate, and more.  The problem is that FFmpeg outputs a carriage-return character (\r) instead of a carriage-return-line-feed (\n) each time it writes these metrics.

In order to parse the FFmpeg metrics with file-tail, we needed to specify a different line-separate character (\r).  This pull-request adds the ability to customize the line-separator when instantiating a file-tail instance.

Example:
```ruby
File::Tail::Logfile.tail(log_path, 
                         :backward => 10,
                         :return_if_eof => false,
                         :line_separator => "\r") do |line|
...
end
```