Add option to include global parameters

[jalopezcar]

The current implementation doesn't take into account the global parameters from each spec.
For example,

swagger: '2.0'
info:
  version: 1.0.0
  ..
parameters:
  CommonPathParameterHeader:
    name: COMMON-PARAMETER-HEADER
    type: string
    in: header
    required: true

I'd like to add a new option, includeParameters. If it's true it would merge the global parameters from all the specs.

It would be implemented in a similar way as includeDefinitions (#46)