Custom rules support

[Anilm3]

Custom rules support, some facts for the reviewer:

• Custom rules live alongside standard rules.
• Custom rules have precedence over ASM rules, however rules with side-effects still have precedence over rules without side-effects.
• Custom rules are sufficient to instantiate the WAF.
• Exclusion filters can affect custom rules.
• Rule overrides can't affect custom rules.

[codecov-commenter]

Codecov Report

Merging #154 (2a25610) into master (e40a3f4) will increase coverage by 0.01%.
The diff coverage is 46.93%.

@@            Coverage Diff             @@
##           master     #154      +/-   ##
==========================================
+ Coverage   64.94%   64.95%   +0.01%     
==========================================
  Files          64       64              
  Lines        3158     3179      +21     
  Branches     1429     1451      +22     
==========================================
+ Hits         2051     2065      +14     
+ Misses        314      308       -6     
- Partials      793      806      +13     

Flag	Coverage Δ
waf_test	64.95% <46.93%> (+0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/collection.hpp	80.00% <0.00%> (-20.00%)	⬇️
src/context.hpp	69.56% <0.00%> (ø)
src/mkmap.hpp	49.09% <ø> (+10.90%)	⬆️
src/parser/specification.hpp	100.00% <ø> (+10.00%)	⬆️
src/rule.cpp	75.00% <ø> (-3.79%)	⬇️
src/ruleset_builder.hpp	85.71% <ø> (ø)
src/parser/parser_v1.cpp	25.25% <25.00%> (+0.51%)	⬆️
src/ruleset.hpp	46.66% <27.27%> (-22.57%)	⬇️
src/collection.cpp	66.66% <33.33%> (ø)
src/context.cpp	60.00% <40.00%> (-2.03%)	⬇️
... and 3 more

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