Add warning filter
Add Mutant::WarningFilter.use
Improve YARD docs
Merge branch 'master' into warning-filter
Use concord 0.1.5 and its super API
Organize helpers methods on ::Mutant

* Use the most central location
* Need for file under lib/mutant/ is gone with new zombifier.
Move parser exetension to non zombification whitelisted file

Zombie::Parser had preference in const lookup but was not compleate.
Use unparser from releases
Do not mutante nthref to gvar
Remove parser monkey patch
Monkeypatch warning in parser away
Fix invalid ASTs on mutating attribute assignments

* Also move index reference / assignment mutators to dedicated file
* Move attribute assignment cases to subclass
* Mutations emitted never had been invalid. Since invalid AST was parsed
  to valid source as unparser trusts the AST.
Normalize ASTs before mutating spec examples

* Reduces smantically unneded empty begin nodes etc.
* Allows to sharp specs.
Do not emit implicit self within op assign on attribute

* This is an invalid AST.
Fix invalid ASTs on match_current_line node
Fix expectations on mutations that affect lvar scope

* It will be a BIG efford to track lvar scope modifications by stuff
  like statement deletion to avoid invalid ASTs.
* Unparser still unparses to valid ASTs in that case.
Whitelist another invalid AST

* Unparses to valid source. Can be addressed by rewriting op_asgn
  mutations to be receiver / context specific.
Remove expectation for invalid AST mutation
Fix overlong line with horrible formatting
Fix some whitespace issues