Add initial value to start method

[SteveAyre]

When start is called it updates the value to zero. The calculations for ETA, speed etc then assume that the progress started from zero. If the bar starts when some progress has been made this can lead to some odd statistics, such as 200T/s transfer rates.

This could be avoided with an optional initial value to the start method, and then use that offset when calculating the rate of change.

[Wistral]

That was what I exactly want to solve... eg. File downloading interrupted and continue to download.