Add start-parameter to progress_bar

[Flole998]

This parameter specifies where the progress_bar starts. It can be used to implement a busy-indicator for example.

[djowel]

There should be a better way. One of the primary rules I adhere to is that the user should not pay for features that he does not use. I might sometimes break that rule if the functionality is used often enough, at least 50% of the time. This is not the case here.

I think this can be better served as a derived class or even a totally separate class which provides the desired behavior, without adding any extra member variable.