Add iterator interface to VideoReader objects

[galenlynch]

I have added an iterator interface to the VideoReader type, making collecting all frames of a video as simple as:

f = openvideo(fname)
imgs = collect(f)

The iterator returns a new array for each frame, as I think mutating the same array on subsequent calls to iterate would be surprising and lead to strange behavior when calling functions such as collect. Note that the VideoReader iterator is mutable, like Channel or Stateful iterators in Julia, and will resume iteration where it was last stopped, instead of at the beginning. I have added some text in the documentation explaining this behavior, and also added some tests.

[galenlynch]

I'm confused by these testing errors.

[codecov[bot]]

Codecov Report

Merging #261 into master will decrease coverage by 3.19%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #261      +/-   ##
==========================================
- Coverage   77.13%   73.94%   -3.20%     
==========================================
  Files          14       14              
  Lines         608      614       +6     
==========================================
- Hits          469      454      -15     
- Misses        139      160      +21     

Impacted Files	Coverage Δ
src/VideoIO.jl	52.27% <ø> (ø)
src/avio.jl	73.80% <100.00%> (-5.36%)	:arrow_down:
src/util.jl	59.25% <0.00%> (-11.12%)	:arrow_down:

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[IanButterworth]

Looks good now. Shall we merge?

[galenlynch]

Fine by me!