GraphQL support / accessor plugin system
I recently added graphql support in [vaex](https://github.com/vaexio/vaex) (another dataframe library), and just added support for pandas as well: https://github.com/vaexio/vaex/pull/446

However, this is only available after importing vaex.graphql, e.g.:
![image](https://user-images.githubusercontent.com/1765949/67106297-9fe84d00-f1ca-11e9-81fa-7d7c2883be4f.png)

In vaex, I use a plugin system using the entry point system, e.g:
```
entry_points={
        'vaex.dataframe.accessor': ['graphql = vaex.graphql:DataFrameAccessorGraphQL'],
    },
```

Is this maybe something pandas can do as well? That would mean that simply installing vaex.graphql would enable this.

Note: in vaex I lazily import the modules, so if `df.graphql` never gets accessed, vaex.graphql will never be imported.
