Support for setting an initial index on stringify() and stringifySynch()

[haschdl]

The implementation of both methods include an instance of Formatted , which in turn is instantiated with a default index=1.
This prevents appending to existing file. In order words, stringify() always generates an output starting from 1.
In my use-case, I translate one SRT “A” onto another SRT “B”. Suppose A has 300 entries.
The automatic translation takes a long-time, so I do it in batches, lets say 8 nodes at a time; and sometimes the process fails. Suppose it fails at the 150th node - when the program restarts, I can detect the last index in the output file and continue from there. However stringify() will generate nodes starting from 1, so the output will have nodes 1..,148,149,150,1,2,…

Adding an overload that accepts a starting index would possibly be a solution.

My work-around at the moment: re-write the entire output file after each batch is processed. It’s not a huge performance hit but makes the process more convoluted.