Allow configuration of the expectedMixins when calling Eloquent::writeEloquentModelHelper()

[challgren]

Summary

I use the plugin Tpetry/PostgresqlEnhanced and it adds some useful Postgres Features to Laravel but the autocomplete in PHPStorm isn't working fully. I did notice the write-mixin option that seems to help with the autocomplete but when using the postgres-enhanced plugin the Builder has more features available. One feature I've been using a lot of is the whereBoolean method on \Tpetry\PostgresqlEnhanced\Query\Builder.

To get this to be added into the autocomplete I was able to modify the expectedMixins array but this wouldn't be a scalable feature.

I'd be happy to contribute a PR unless there's a better way to accomplish what I want.