Refactor and add docs

[codecov[bot]]

Codecov Report

Patch coverage: 73.03% and project coverage change: +3.08% :tada:

Comparison is base (3ad2d84) 6.88% compared to head (05d4a98) 9.97%.

:exclamation: Current head 05d4a98 differs from pull request most recent head 1956c69. Consider uploading reports for the commit 1956c69 to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #101 +/- ## ========================================= + Coverage 6.88% 9.97% +3.08% ========================================= Files 15 17 +2 Lines 2366 2375 +9 ========================================= + Hits 163 237 +74 + Misses 2203 2138 -65 ``` | [Files Changed](https://app.codecov.io/gh/JuliaIO/GZip.jl/pull/101?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaIO) | Coverage Δ | | |---|---|---| | [src/GZip.jl](https://app.codecov.io/gh/JuliaIO/GZip.jl/pull/101?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaIO#diff-c3JjL0daaXAuamw=) | `100.00% <ø> (+27.52%)` | :arrow_up: | | [src/zlib.jl](https://app.codecov.io/gh/JuliaIO/GZip.jl/pull/101?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaIO#diff-c3JjL3psaWIuamw=) | `0.00% <0.00%> (ø)` | | | [src/gz.jl](https://app.codecov.io/gh/JuliaIO/GZip.jl/pull/101?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaIO#diff-c3JjL2d6Lmps) | `73.86% <73.86%> (ø)` | | ... and [2 files with indirect coverage changes](https://app.codecov.io/gh/JuliaIO/GZip.jl/pull/101/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaIO)

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