add decoder

[johnnychen94]

Although decoder can be easily implemented in pure Julia. For completeness and consistency, here I just delegate all these operations to the libsixel

[codecov]

Codecov Report

Merging #7 (c8e933d) into master (607847e) will increase coverage by 14.34%.
The diff coverage is 93.75%.

@@             Coverage Diff             @@
##           master       #7       +/-   ##
===========================================
+ Coverage   24.42%   38.76%   +14.34%     
===========================================
  Files           7        9        +2     
  Lines         262      325       +63     
===========================================
+ Hits           64      126       +62     
- Misses        198      199        +1     

Impacted Files	Coverage Δ
src/encoder.jl	85.71% <ø> (ø)
src/interface.jl	0.00% <ø> (ø)
src/decoder.jl	91.48% <91.48%> (ø)
src/Sixel.jl	100.00% <100.00%> (ø)
src/backend/libsixel/decoder.jl	100.00% <100.00%> (ø)
src/backend/libsixel/CModule.jl	4.45% <0.00%> (+1.27%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 607847e...c8e933d. Read the comment docs.