Allow forced UTF-8 encoding for --export-config

[pdxlocations]

This PR changes the --export-config command to save a UTF-8 encoded file instead of printing to stdout. Allows the legacy use of > to redirect to a file.

To fix incorrect encoding on Windows Powershell:

meshtastic --export-config file.yaml

Also allow:

meshtastic --export-config > file.yaml

closes #728

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 9 lines in your changes missing coverage. Please review.

Project coverage is 59.78%. Comparing base (c55f1ef) to head (0ae23ee).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
meshtastic/__main__.py	0.00%	9 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #798      +/-   ##
==========================================
- Coverage   59.90%   59.78%   -0.13%     
==========================================
  Files          24       24              
  Lines        4163     4170       +7     
==========================================
- Hits         2494     2493       -1     
- Misses       1669     1677       +8     

Flag	Coverage Δ
unittests	59.78% <0.00%> (-0.13%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.