Add skip argument to search funcs

[jlumpe]

Add skip argument to find and find_nearest, which is a callback f(i::Int)::Bool indicating whether to exclude data point with index i from the results.

Tests require #5 be merged first.

[altre]

I'm not sure if this is a good thing to include in the API. Can't you just filter the result? Or do you have a special use case?

[jlumpe]

For find you can definitely just filter the result. find_nearest is more complicated though because you don't know ahead of time how many data points you'll have to go through before you find k that have passed the filter. It's relevant for my uses because my data points are classified and I want to find the closest k for a point outside of its assigned class. There are other common use cases as well - for example Theiler windows in time series analysis. Basically it boils down to excluding the data points you know a priori will be close to your query in order to focus on the unexpected ones.

This is implemented in other nearest-neighbor libraries, like NearestNeighbors.jl.

[jlumpe]

Tests implemented, so should be good to merge if you think it's appropriate.

[altre]

Makes sense to me. Thanks for the PR.