Added a fix for issue #131

Description

Added a single line that wipes out the default channel rule when adding others

Related Issue

#131

Motivation and Context

Able to restrict commands in slack channels

How Has This Been Tested?

Confirmed that when adding rules the created configuration did not include the default channel rule.
Added rules and tested in own slack workspace.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.