feat(cli): Add ls command to list files and sizes in a snapshot

[nellh]

Adds a simple ls command to list a snapshots files.

> openneuro ls ds000001 -s 1.0.0
CHANGES 286
README  1175
dataset_description.json        615
participants.tsv        216
task-balloonanalogrisktask_bold.json    73
sub-01/anat/sub-01_T1w.nii.gz   5663237
sub-01/anat/sub-01_inplaneT2.nii.gz     669578
sub-01/func/sub-01_task-balloonanalogrisktask_run-01_bold.nii.gz        47241449
sub-01/func/sub-01_task-balloonanalogrisktask_run-01_events.tsv 8610
sub-01/func/sub-01_task-balloonanalogrisktask_run-02_bold.nii.gz        47282515
sub-01/func/sub-01_task-balloonanalogrisktask_run-02_events.tsv 8568
sub-01/func/sub-01_task-balloonanalogrisktask_run-03_bold.nii.gz        47347339
sub-01/func/sub-01_task-balloonanalogrisktask_run-03_events.tsv 8132
...

[codecov[bot]]

Codecov Report

Merging #2689 (eab8adc) into master (b3c6205) will decrease coverage by 0.04%. The diff coverage is 10.93%.

@@            Coverage Diff             @@
##           master    #2689      +/-   ##
==========================================
- Coverage   39.91%   39.86%   -0.05%     
==========================================
  Files         563      564       +1     
  Lines       34377    34437      +60     
  Branches      914      915       +1     
==========================================
+ Hits        13722    13729       +7     
- Misses      20529    20581      +52     
- Partials      126      127       +1     

Impacted Files	Coverage Δ
packages/openneuro-cli/src/actions.js	0.00% <0.00%> (ø)
packages/openneuro-cli/src/cli.js	0.00% <0.00%> (ø)
packages/openneuro-cli/src/ls.js	0.00% <0.00%> (ø)
packages/openneuro-cli/src/datasets.js	54.88% <35.00%> (-2.01%)	:arrow_down:

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