Add Rake/ConstantDefinitionInTask cop

[corsonknowles]

What

Adds a new cop, Rake/ConstantDefinitionInTask, that registers an offense
for a constant assignment inside a task or namespace.

# bad
task :foo do
  CONST = 1
end

# bad
namespace :foo do
  CONST = 1
end

# good - It is also defined to the top level, but it looks expected behavior.
CONST = 1
task :foo do
end

Why

A constant assigned inside a task or namespace block is defined to the
top level, not scoped to the block. That is the same surprise the existing
cops already cover for methods and classes:

• Rake/MethodDefinitionInTask for methods
• Rake/ClassDefinitionInTask for classes and modules
• Rake/ConstantDefinitionInTask (new) for plain constant assignments

Rake files are also sometimes loaded rather than required, so a constant
that leaks to the top level can produce "already initialized constant"
warnings on reload.

The cop reuses Helper::TaskDefinition, and like Rake/ClassDefinitionInTask
it skips nodes inside a class, module, or Class.new body via
Helper::ClassDefinition.in_class_definition?, because those constants are
not defined to the top level.

Details

Adds the cop and its spec, a config/default.yml entry (Enabled: pending,
VersionAdded: '<<next>>'), the rake_cops.rb require, and a CHANGELOG
entry. bundle exec rake is green.