System to mark "auto-populatable" script options in the web API

[bertfrees]

Related to daisy/pipeline-ui#345.

Examples of options that could be auto-populated:

• Include TTS log
• Lexicon
• Image resolution
• Template for DTBook to ODT
• ...

In fact most of the options are candidates for auto-population (it's just not always clear whether it's better to auto-populate them based on previous jobs, or to have a settable default).

Examples of options that should not be auto-populated (i.e. for which the default makes more sense than the value set by the previous job) are:

• Braille code (the default is to choose a braille code based on the langauge of the input document)
• ...