Lazy mode and streaming optimisations

This release includes a number of optimisations, deprecations, and API changes:

• -lazy flag added to the attack command. This enables reading infinite streams of input targets.
• Hit results are now streamed out of attack as soon as they arrive. Before, all hits would have to finish before sending the results out.
• The report command can now read its inputs concurrently and is responsible for aggregating and sorting them.
• Removed order attack option as well as the associated library code. Randomizing the order of targets can be easily achieved outside of Vegeta.
• Removed the attached Encode, Decode and Sort methods of the Results type. These were just clutter passthroughs.

Of course, these changes are reflected in the library too. For instance, whereas before you'd invoke Attack with a slice of Targets, now you need to provide a Targeter. Please refer to the updated godocs.

This is a rather massive release in terms of code changes but the improvements are worth it. Please report any bugs or inconsistencies.

Cheers,
Tomás