Support Json annotations

[kennymckormick]

Support the new json annotation format, which is more scalable than txt files:

1. You can use the command python tools/data/anno_txt2json.py annotation.txt to convert it from txt to json.
2. The dataset can automatically detect json annotation list and load it.
3. When the repo decides to switch from txt to json, just change the default option of output_format in build_file_list.py to 'json'

[codecov]

Codecov Report

Merging #119 into master will decrease coverage by 0.37%.
The diff coverage is 13.04%.

@@            Coverage Diff             @@
##           master     #119      +/-   ##
==========================================
- Coverage   85.53%   85.15%   -0.38%     
==========================================
  Files          76       76              
  Lines        4155     4177      +22     
  Branches      662      667       +5     
==========================================
+ Hits         3554     3557       +3     
- Misses        494      512      +18     
- Partials      107      108       +1     

Flag	Coverage Δ
#unittests	85.15% <13.04%> (-0.38%)	⬇️

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Impacted Files	Coverage Δ
mmaction/datasets/rawframe_dataset.py	85.85% <0.00%> (-1.78%)	⬇️
mmaction/datasets/video_dataset.py	82.53% <0.00%> (-2.71%)	⬇️
mmaction/models/backbones/resnet_tsm.py	95.08% <ø> (ø)
mmaction/datasets/base.py	67.24% <11.11%> (-25.26%)	⬇️
mmaction/models/backbones/resnet3d_csn.py	100.00% <100.00%> (+2.63%)	⬆️

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[dreamerlin]

You can remove the "WIP" label when the PR is ready