(#476) Add validation of input file path handler

[AdmiringWorm]

Description

This updates how arguments are parsed when attempting to create a new release. This disallows explicitly the use of a milestone, and an input file path in the same call.

Related Issue

fixes #476

Motivation and Context

This is done as there was confusion on why specifying a input file path was not using the release notes specified. This should make it clearer to the caller what the issue is.

How Has This Been Tested?

1. Run grm create --milestone 0.5.0 -i some-relase-notes.txt with additional valid arguments.
2. Ensure an exception is thrown explaining that these two arguments can not be used together.

Screenshots (if appropriate):

Checklist:

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

[gep13]

@AdmiringWorm thank you very much for taking the time to fix this up!