Support for attaching custom fixers that are defined separetly to the plugin

[johnmathews]

It would be useful to be able to specify a directory outside the Plugin where ALE can look for and register custom fixers.

In order to register a new fixer it is necessary to edit files that are part of the plugin package. This makes it cumbersome to create custom fixers and register them in multiple environments.

[hsanson]

Is possible to define custom fixers on your vim configuration without modifying ALE itself:

:h ale#fix#registry#Add()