source bash config from existing bash config to make rolling back changes easier, use posix-compliant echo

[cartercanedy]

Changes

• source bash config from this repo from existing bashrc rather than overwriting
• detect if the new bash config is already being sourced, don't append if it's been found

Rationale

There's been multiple people that have used this bash configuration setup from the linutil project that have been left without a way to rollback the changes.

Issues reported:

• ChrisTitusTech/linutil#260
• ChrisTitusTech/linutil#240

Pulling in these changes should make it pretty trivial to revert any modifications to the bash prompt