Add category to rules' meta objects

[lolgab]

This PR adds category and description inside the meta object.
I didn't have too much creativity for the no-mixed-html description. If you have better ideas let me know and I update the PR.

[codecov-io]

Codecov Report

Merging #6 into master will increase coverage by <.01%.
The diff coverage is 98.51%.

@@            Coverage Diff             @@
##           master       #6      +/-   ##
==========================================
+ Coverage   98.55%   98.55%   +<.01%     
==========================================
  Files           5        5              
  Lines         414      415       +1     
  Branches      149      149              
==========================================
+ Hits          408      409       +1     
  Misses          6        6

Impacted Files	Coverage Δ
lib/rules/no-location-href-assign.js	100% <ø> (ø)	⬆️
lib/rules/no-mixed-html.js	98.54% <98.51%> (ø)	⬆️

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