Add DigitMap keyword for intuitively inserting digit map (dial plan)

[richard-kung]

• Allow to insert digit map anywhere instead of appending it
• Allow to specify input dial plan filename directly for easily managing mutiple dial plans
• Change default decoded dialplan.txt to dialplan_output.txt to prevent accidentally overwrite input file

Example files:

• examples/packetcable_digitmap.txt (line 40)
• examples/packetcable_digitmap.cfg

[rlaager]

@richard-kung Just so you're aware, I don't use this software any more and know very little about its codebase. So while I gave the project repository a home, my ability to review changes is limited.

I did give this a read through. I don't see anything obviously wrong here at first glance.

[richard-kung]

@rlaager I will handle Cable modem/eMTA projects for at least one year and will use this tool. If you need some helps, feel free to ask.