Customize field names without alias

[clinta]

Is your feature request related to a problem? Please describe.
By default ugly names will be generated if the field name contains an underscore. This can be worked around using aliases as documented, but this has limitations. If the server operator has configured max aliases, queries will fail when they exceed this.

Describe the solution you'd like
The ability to customize the field names using a @genqlient directive as an alternative to aliases.