Assist escapeReservedWord with custom mappings

[AlexNolasco]

PR checklist

• Read the contribution guildelines.
• Ran the shell/batch script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates)
• Filed the PR against the correct branch: master for non-breaking changes and 2.3.0 branch for breaking (non-backward compatible) changes.

Description of the PR

The escapeReservedWord method does its job fairly well, but in some cases "reserved words" make some properties look a bit odd in some languages.

The method escapeReservedWord could be assisted through an additional command line parameter (reserved-words-mappings).

In the case of Objective-C, the definition below would create a property named _id which could mistakenly be interpreted as an iVar.

definitions:
  Pet:
    required:
      - id
    properties:
      id:
        type: integer
        format: int64
     name: 
        type: string

which in turn yields

@interface SWGPet : SWGObject
@property(nonatomic) NSNumber* _id;
@property(nonatomic) NSString* name;
@end

The escapeReservedWord method could look up what _id should be substituted with instead.

For example,
java -jar swagger-codegen-cli.jar generate -i pet.yaml -l objc --reserved-words-mappings id=identifier -o output

The new output

@interface SWGPet : SWGObject
@property(nonatomic) NSNumber* identifier;
@property(nonatomic) NSString* name;
@property(nonatomic) NSString* tag;
@end

GoLang can also benefit from this, for example

  Car:
    required:
      - id
      - name
      - range
    properties:
      range:
        type: array
        items:  
          type: integer
      id:
        type: integer
        format: int64
      name:
        type: string

Instead of this

type Car struct {
	Range_ []int32 `json:"range,omit empty"
	Id int64 `json:"id,omit empty"
	Name string `json:"name,omit empty"
}

It could generate

type Car struct {
	Ranges []int32 `json:"range,omit empty"
	Id int64 `json:"id,omit empty"
	Name string `json:"name,omit empty"
}

by using
java -jar swagger-codegen-cli.jar generate -i car.yaml -l go --reserved-words-mappings Range=Ranges -o output

[wing328]

@AlexanderN thanks for the enhancement. I'm sure other Swagger Codegen users will find this new feature useful in customizing the output.