allow density/normed argument

[bastibe]

Thank you for your cool package!

numpy.histogram and numpy.histogram2d support the normed argument, which divides counts by the number of samples and the bin width/area. It would be great if fast-histogram could support this as well.

[astrofrog]

@bastibe - would you be interested in opening a pull request to add these options? I think that the default values that Numpy use mean that this won't add much overhead for the default use case.

[bastibe]

Sure, I can do that. It will take a few days, though.