add force flag

[rizowski]

Related issue:

closes #12

Why did you make this change

To allow the user to be able to force delete branches if there is no way to verify the upstream has the changes from a local branch.

What have you done in this PR

Added a --force flag which allows users to be able to acknowledge they want to delete the branches regardless of the repercussions.

What was left to do

nothing

Dependencies

none

Checklist:

• [x] My code follows the style of this project
• [x] I have performed a self-review of my own code
• [x] I have have tested my code locally to prove my fix is effective or that my feature works
• [x] I have made corresponding changes to the README.md