Part number and parts count added

[vadzay]

Hello!

In some cases when tailer splits line in parts there is no way to find out if I received part of message or whole message if len(line.Text) == tail.MaxLineSize.

I've added part number and total parts to each message sent to tail.Lines channel.

[srid]

thanks for the pull request.

i took a look at the diff, and don't feel very sure about adding this. generally i like to keep the Line struct smaller, which would make sense if you are transferring say millions of lines as part of some log streaming application. the two int64 fields you added -- PartNum and Parts -- are going to be empty most of the time (as it is uncommon for lines to be very long), thus wasting so much space in any app that is dealing with large streams of logs.

you can reduce that to just one field continuation bool that indicates whether a line is a continuation of the previous line, but still - i wonder if we can do away with that as well.

[Nino-K]

@vadzay I agree with @srid going to close this PR, thanks for your contribution.