Improve the xmltojson.py script

[rudolfbyker]

• Follow PEPs.
• Clean up imports.
• Separation of concerns and better code reuse by splitting up into functions.
• Make silent by default, but add 'verbose' flag.
• Use a context manager when writing a file, to prevent leaking file descriptors.

[rudolfbyker]

@dstillman @northword would you prefer that I split this into smaller PRs?