avrogo generates non-idiomatic names

Names produced by `avrogo` do not follow idiomatic Go capitalisation/initialisation rules. (e.g. use `ID` rather than `id` or `Id`).

This can be reproduced with the following schema
```
{
"name": "User",
"type": "record",
"fields": [
{
"name": "userID",
"type": {
"type": "string",
"logicalType": "uuid"
}
}
]
}
```

`avrogo` generates
```
type User struct {
Userid uuid.UUID `json:"userID"`
}
```

but an idiomatic representation would be
```
type User struct {
UserID uuid.UUID `json:"userID"`
}
```

There are third-party libraries available to achieve this (e.g. [github.com/hamba/avro](http://github.com/hamba/avro) uses [github.com/ettle/strcase](http://github.com/ettle/strcase)'s PascalCase).

If this were implemented then it would be helpful to allow additional initialisms with command line arguments. For example in my use case I have schemas referring to `MW` (megawatts) and would like that initialised too.

I'm happy to work on this but I want to check if the change would be accepted before starting.