Add pre-jgitflow function

[ppalazon]

Add pre-jgitflow function

Description

Prepare a function to be executed before any jgitflow command

Motivation and Context

I have to check if I had access to my codeartifact repository before
any jgitflow command. With this improvement I can implement this function
on my ~/.bashrc file, and it would be executed before any jgitflow command.

function pre-jgitflow {
    source aws-auth-codeartifact
    return $(test -n "$CODEARTIFACT_AUTH_TOKEN")
}

How Has This Been Tested?

I've been testing this process for a few months, but I don't know how to write
a unit test on this framework.

Screenshots (if appropriate):

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.
• If my change requires a change to the documentation, I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• If I have added a new file, I also added it to clean_files.txt and formatted it using lint_clean_files.sh.
• I have added tests to cover my changes, and all the new and existing tests pass.

[seefood]

seems legit.