Add default input & context data

[oliverde8]

Most ETL executions require data as an input(even if empty) and will also often require some context.

1. Add a new input section that will be read as the default value if no input is given. This is optional!

input:
    default: 
        value: []

2. Add a new context section; context can also be appended to make execution easier.

context:
  default: 
    value: {myKey: 1}
    behaviour: merge # or replace

If behaviour is merge then the parameters inputed in the command line will be merged with those in the default. In replace mode the input will completely replace the default value. The default behaviour is append.

Both these sections needs to be optional not to create a BC break.

[oliverde8]

default input was added.