Make features linkable and reference them in the docs

[Mingun]

I not found a way to link the crate root -- it seems that this is impossible right now

[codecov-commenter]

Codecov Report

Merging #649 (75a96fc) into master (f4af33e) will not change coverage.
The diff coverage is 80.00%.

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@@           Coverage Diff           @@
##           master     #649   +/-   ##
=======================================
  Coverage   64.36%   64.36%           
=======================================
  Files          36       36           
  Lines       17343    17343           
=======================================
  Hits        11162    11162           
  Misses       6181     6181           

Flag	Coverage Δ
unittests	64.36% <80.00%> (ø)

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Files Changed	Coverage Δ
src/de/map.rs	83.41% <ø> (ø)
src/encoding.rs	100.00% <ø> (ø)
src/errors.rs	10.43% <ø> (ø)
src/escapei.rs	13.24% <ø> (ø)
src/events/attributes.rs	93.56% <ø> (ø)
src/reader/mod.rs	95.04% <ø> (ø)
src/reader/parser.rs	98.70% <ø> (ø)
src/utils.rs	67.36% <ø> (ø)
src/lib.rs	18.63% <80.00%> (ø)