apply: make diff support n time steps lag

[iblislin]

julia> diff(cl, 5)
495x1 TimeSeries.TimeArray{Float64,1,Date,Array{Float64,1}} 2000-01-10 to 2001-12-31
│            │ Close  │
├────────────┼────────┤
│ 2000-01-10 │ -14.19 │
│ 2000-01-11 │ -9.75  │
│ 2000-01-12 │ -16.81 │
   ⋮
│ 2001-12-27 │ 0.45   │
│ 2001-12-28 │ 1.76   │
│ 2001-12-31 │ 0.9    │

[codecov-io]

Codecov Report

Merging #362 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #362   +/-   ##
=======================================
  Coverage   86.22%   86.22%           
=======================================
  Files          10       10           
  Lines         479      479           
=======================================
  Hits          413      413           
  Misses         66       66

Impacted Files	Coverage Δ
src/apply.jl	90.9% <100%> (ø)	⬆️

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