Support constants in input mapping

[ctmay4]

Input mapping currently looks like this:

"input_mapping": [
    {  "from": "ajcc7_t", "to": "t" }
]

This example temporarily puts the context value contained in "ajcc_t" into "t" before processing a table. This allows tables to be reused with different inputs. The need has arisen to use constant values instead of existing keys. This is not difficult, however a good syntax needs to be decided on. Static strings could be quoted. That would look like this:

"input_mapping": [
    {  "from": "\"constant\"", "to": "t" }
]

[ctmay4]

To be clear, constants would be allowed for input_mapping.from and output_mapping.to.