Release/v0.6.0

[jkawamoto]

• Use newer API to join paths of a URL #33
• Initialize a client in before function #34
• Migrate from go-multierror to errors #35
• Upload command accepts multiple files #36
• Simplify client package #37
• Download command accepts multiple files and lists #38

Closes #13.

[codecov[bot]]

Codecov Report

Merging #39 (a01ca51) into main (36d1ad5) will increase coverage by 17.37%. The diff coverage is 55.46%.

@@             Coverage Diff             @@
##             main      #39       +/-   ##
===========================================
+ Coverage   42.24%   59.61%   +17.37%     
===========================================
  Files          13       10        -3     
  Lines         303      468      +165     
===========================================
+ Hits          128      279      +151     
+ Misses        166      162        -4     
- Partials        9       27       +18     

Impacted Files	Coverage Δ
error.go	100.00% <ø> (+21.42%)	:arrow_up:
cmd/pd/internal/testutil/auth.go	35.71% <35.71%> (ø)
cmd/pd/command/mock/mock_service.go	44.20% <44.20%> (ø)
cmd/pd/command/download.go	50.50% <50.00%> (+50.50%)	:arrow_up:
cmd/pd/main.go	75.86% <57.14%> (+75.86%)	:arrow_up:
pixeldrain.go	70.00% <67.85%> (-30.00%)	:arrow_down:
cmd/pd/command/upload.go	76.00% <78.46%> (+76.00%)	:arrow_up:
client/pixeldrain_api_client.go	77.50% <100.00%> (+1.18%)	:arrow_up:
cmd/pd/auth/context.go	100.00% <100.00%> (ø)
transport.go	54.16% <100.00%> (ø)

... and 1 file with indirect coverage changes