Allow configuration of Prettier options

[dev-nicolaos]

It would be great to have a way to set prettier options when using deno fmt. Up until a recent release (I'm guessing 0.18, but didn't notice till 0.19) I was able to run deno fmt files -- --prettier-option-1 true, though this was still a less than ideal process. My prefered method would be to have a prettier config file and for deno to either pick it up automatically or allow a flag to pass in a path to said file.