v1.0.0.b More customisation, error handling

Upstream branch options, merge and push arguments

Changelog

More customisation, because why not?

• Upstream branch customisation (ever thought upstream could change branch name?)
• Actually fails if something goes wrong (my code can't be wrong amirite)
• Git merge arguments (now you can --ff --allow-unrelated-histories your way home)
• Git push arguments

Usage Example

• Refers to README.md
• Refers to janky example

Full Changelog: v0.1.2...v1.0.0.b