Rewrite bashrc sourcing logic

* Factor out AWS and OSX toggles from files and consolidate to bashrc
* Organize bashrc into clear specific sections and reorder sourcing
* Move OSX commands to existing source file
* Add a new source file for AWS
Remove all private and host-specific references

This is prep work for implementing host-specific settings
and secrets via a new private repo and rcm hooks.
Add new all new gibo .gitignore rules
Add sourcing for global and brew-specific PATHs