Add colored and git-aware `PS1` prompt

- Change colors based on git status (clean/untracked files, changes)
- Show `HEAD` when git head is detached
Symlinks link to relative paths

By using relative rather than absolute paths, home folders shared
between different hosts no longer breaks if the absolute path to home
folders differ.

If there is no common path between the home directory and dotfiles
folder, installer will revert to old behavior.
Add irbrc

- Require `irb/completion` for tab completion in console
Extract resource list from script to manifest file