Add export option to tesla-keygen

[sethterashima]

Description

Having exportable private keys isn't ideal, but is sometimes needed. This adds an "export" subcommand to tesla-keygen.

Fixes #193 #258

Type of change

Please select all options that apply to this change:

• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Bug fix (non-breaking change which fixes an issue)
• Documentation update

Checklist:

Confirm you have completed the following steps:

• My code follows the style of this project.
• I have performed a self-review of my code.
• I have made corresponding updates to the documentation.
• I have added/updated unit tests to cover my changes.