Add possibility to configure own path to `/api-docs/swagger.json`

[vkatsuba]

The /api-docs/swagger.json is hardcoded at https://github.com/inaka/cowboy_swagger/blob/2.7.0/priv/swagger/swagger-initializer.js#L6 and it lead to issues if few dependencies in the project are use this great library for creating Swagger. It will be nice if path to /swagger.json can be configured or at least depended by name of application where this library is used.