Simplify passing unsharp opions

[puzrin]

Currently users are expeced to modify processing with ease. But probably passing unsharp options need something special.

At first glance, possible alternatives are:

• pass unsharp params the same way as { max: ... }
  • need to limit list of allowed params (currently unsharpAmount, unsharpRadius, unsharpThreshold).
• pass unsharp params as { max: ..., pica_opts: ... } - separate object to merge on pica call.
  • if user doesn't knows about pica, this looks unnatural.
• suggest to pass pica into construcor with overrien .resize()
  • mad science :)

Additional quesions:

• do we need to pass pica's alpha? It's logic is hardcoded now. Can be changed with _.transform() override only.
• do we need to pass jpeg quality option? Can be changed with .toBlob() override, not a big deal.

[puzrin]

4b11846