add method "add_video" to SummaryWriter

[raahii]

I implemented the feature of logging a video clip as gif images to tensorboard. I used this pull request as reference.

[codecov-io]

Codecov Report

Merging #2 into master will decrease coverage by 0.02%.
The diff coverage is 86.66%.

@@            Coverage Diff             @@
##           master       #2      +/-   ##
==========================================
- Coverage   87.07%   87.04%   -0.03%     
==========================================
  Files          25       25              
  Lines        1052     1096      +44     
==========================================
+ Hits          916      954      +38     
- Misses        136      142       +6

Impacted Files	Coverage Δ
setup.py	0% <ø> (ø)	⬆️
tb_chainer/writer.py	89.74% <100%> (+0.17%)	⬆️
examples/demo.py	100% <100%> (ø)	⬆️
tb_chainer/summary.py	91.17% <85%> (-2.58%)	⬇️

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[neka-nat]

Thank you, raahii!
I think that it is a very good function.
But the coverage has declined, so can you add test?

[neka-nat]

Thanks!