Fix stdout output

[x11x]

Properly writes JSON output to the file, instead of writing the __repr__ of a Python bytes literal containing the JSON.

Also changes decryptBitwardenJSON to return a dictionary instead of JSON-encoded string, and adds write_json function to handle JSON-encoded output (using json.dumpinstead ofprintorTextIO.write`).

Fixes #24

[x11x]

To clarify how this handles output encoding:

This adds an --output-encoding command-line parameter to BitwardenDecrypt.py

On Windows:
- uses --output-encoding if specified
- otherwise uses PYTHONIOENCODING environment variable if specified (leaves encoding=None to let Python handle this for us)
- defaults to "utf-8" to help Windows users whose default encoding is e.g. "cp1252" which they might not have expected

On other platforms:
- uses --output-encoding if specified
- does not enforce a default (uses encoding=None to let Python use the default encoding it has determined)