Emit position of actual file size when watching

[th0m4s]

When starting to read a file from a specific position, I needed to know where to display a button to the user to load the start of the file, but I used a second fs.stat which led to some weird behaviours if the file is being written rapidly.

To counter that, I created an event, watch, which is emitted when the file is being read at a specific position (in the watch function), so I could use that number later to retrieve the start of the file.

I didn't find another idea for the name, be pleased to rename it if necessary.

Thanks in advance