Allow methods to be excluded from MethodLength by name

This feature allows you to exclude certain methods from `Metrics/MethodLength` using the `IgnoredMethods` configuration:

```yaml
IgnoredMethods:
  - "MyModule::MyClass#my_method"
  - AnotherClass.my_method
```

Class methods are denoted by `.method_name` and instance methods by `#method_name`. This could probably be generalized and extended to take the place of using comments for disabling cops inside source code.

This feature was inspired by how Cane handles [ignoring methods](https://github.com/square/cane/blob/master/lib/cane/abc_check.rb#L26).