Add an option to skip track size estimate

[Otiel]

It will save some time when you're trying to download lots of albums, plus it should help with the Bandcamp anti-spam filter. If the option is checked, the progress should be calculated on the number of tracks instead of their size.

Suggested by @zontargs on #26.

[ajsnyde]

Let me know if you're working on this - I might implement that enhancement for you..

[Otiel]

I am not at the moment. You can take this task if you want. Thanks!

[ajsnyde]

I've completed an option that allows for this:
ajsnyde@f914011

[Otiel]

Thanks @ajsnyde! Merged and included in the new release v0.1.9.2.