implement #2241 (custom config file during install)

[mvk]

• update install.sh/uninstall.sh
• update README.md

Description

• added handling of overriding of CONFIG_FILE if BASH_IT_CONFIG_FILE is passed to install.sh/uninstall.sh

Motivation and Context

this allows not touching user's bash config file, making it easier to detect regressions or compare 2 config files.

#2241

How Has This Been Tested?

the change is pretty trivial, ran BASH_IT_CONFIG_FILE=.bashrc_bash_it ./install.sh resulted in created fle, bashrc untouched.

linux: rhel7, ubuntu 22, mac.

doubt there are any other side-effects, please review though.

Screenshots (if appropriate):

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x ] My code follows the code style of this project.
• [x] If my change requires a change to the documentation, I have updated the documentation accordingly.
• [x] 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.