Add optional filter predicates to intersect, and a findfirst function

[dcjones]

See https://github.com/BioJulia/Bio.jl/issues/471 for discussion.

Let's intersection iterators be arbitrarily filtered, e.g. intersect(t1, t2, (a,b) -> a.value == b.value).

[codecov-io]

Codecov Report

Merging #28 into master will increase coverage by 1.54%. The diff coverage is 92.1%.

@@            Coverage Diff             @@
##           master      #28      +/-   ##
==========================================
+ Coverage   91.61%   93.15%   +1.54%     
==========================================
  Files           3        3              
  Lines         775      789      +14     
==========================================
+ Hits          710      735      +25     
+ Misses         65       54      -11

Impacted Files	Coverage Δ
src/IntervalTrees.jl	94.5% <92.1%> (+1.78%)	:arrow_up:

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[TransGirlCodes]

LGTM! :+1: