Add program option to explicitly show the progress bar

[cristicc]

Hi,

I added an option -p to enable explicitly the display of the dotted progress bar, by default it won't be shown.

The rationale is that, with this feature always enabled, it will pollute the terminal where the program was launched without bringing much value. For monitoring the activity I think -d (given one time) is a better approach, anyway.

Additionally, I have updated the usage info to group width & height options under a single optional expression (i.e. [-w <width> -h <height>]) since they cannot be used separately.

Thanks for the great work,
Cristi

[phlash]

Looks good to me, merging. @cristicc FYI we are looking to refactor the debug/display parts in the experimental branch, along with more dynamic controls (on/off, flip, resize, change camera devices...) that would be useful in a live presentation situation where we cannot drop and restart the virtual camera, thus we cannot restart this app.