add greyscale option

[leonnleite]

Hi Dear,

this PR will add a greyscale option

{
...
  greyscale: false
...
}

when the flag is false will be like this:

by default the flag is true

this issue will reported here
#5

[desandro]

Thank you for this contribution. I ended up resolving this request using shades option. See #5 (comment)