Teach formulas about their id

This allows writing formulas that can adapt depending on what script
they're in.

For example, when using a configuration as in http://www.plomino.net/how-to/configuration-forms-and-documents
the fields could get their default values like this:

    #Plomino import libConfig
    ignore, ignore, rest = script_id.split('_', 2)
    field_id, ignore = rest.rsplit('_', 1) 
    return libConfig_getValuesAsDict(field_id).keys()