add support for checking out specific Git branch

Add support for checking out a specific Git branch during a temporary or permanent clone:

[x] optional parameters --branch and -b can specify branch name
[x] also an optional second argument "BRANCH" can specify branch name too (to make using git-peak-extended easier with Bash aliases)
[ ] ensure branch exists before attempting to check it out, if not just checkout default branch and print warning message
[x] will likely live around here: https://github.com/toozej/git-peak-extended/blob/main/git-peak-extended#L204
[x] update README with parameter and argument usage
[x] update README with an alias for checking out a repo with a given branch

toozej / git-peak-extended