Add support for getting ordered blocks

[distbit0]

I have been using this library in one of my projects and it is working perfectly but I have ran into a really big roadblock: I need the blocks that your library returns to be ordered as my code requires this to work. Could you please implement this when you get a chance of even tell me how to implment it myself. Thanks so much for building this! :)

[osancus]

Hi can you please add a sample python snippet explaining how to read blocks in order. I am trying to read all blocks in order from all blk files (blk00000.dat, blk00001.dat, ....)

[alecalve]

By default, get_unordered_blocks will return them in the order they are in the files (blk00000.dat, blk00001.dat, ....) but the order in those files is not always following the order of the main chain (block #0, etc...)

[osancus]

@alecalve Thanks for responding. Firstly, thanks for the great work and I think i wasn't able to explain well. I understand that the files are read in order but i wanted to have a sample code for @asutoshpalai work to read blocks in order.