add encoder

[johnnychen94]

Preview:

Todo:

• docs
• test
• Features
  • io redirection so that we can appropriately test the functionalities in CI (WIP: use low level sixel_encode #2)
  • expose encoder control flags as keyword arguments (currently only transpose)
  • query terminal information
  • default palette might be runtime dependent. Probably we should not hardencode them in default_palette.
• Nice-to-have
  • gif and video support. (5bd3f30) (reverted in 45a7ec3, will be added to ImageInTerminal)
  • better support for Gray{Bool} type.
  • AGray{N0f8} and GrayA{N0f8} are currently converted to ARGB.

[codecov]

Codecov Report

Merging #1 (63cbd9d) into master (2876d7c) will increase coverage by 24.33%.
The diff coverage is 24.33%.

@@             Coverage Diff             @@
##           master       #1       +/-   ##
===========================================
+ Coverage        0   24.33%   +24.33%     
===========================================
  Files           0        7        +7     
  Lines           0      263      +263     
===========================================
+ Hits            0       64       +64     
- Misses          0      199      +199     

Impacted Files	Coverage Δ
src/interface.jl	0.00% <0.00%> (ø)
src/terminaltools.jl	0.00% <0.00%> (ø)
src/backend/libsixel/CModule.jl	3.18% <3.18%> (ø)
src/backend/libsixel/encoder.jl	72.22% <72.22%> (ø)
src/Sixel.jl	80.00% <80.00%> (ø)
src/encoder.jl	85.71% <85.71%> (ø)
src/backend/libsixel/types.jl	100.00% <100.00%> (ø)
... and 4 more

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[johnnychen94]

After playing with this for over a week, I'm now quite satisfied with the current design. Hence I'm going to make this the initial commit to the master.

Windows is currently broken (#5), I'll try to fix that but can't promise anything (as GSoC 2021 is about to start)