Add local file example to the usage section of the README

I can't believe we didn't have this before.

Closes #85.
Remove unreachable and now unused quiet flag for fresh_install
Treat unset variables as an error when performing parameter expansion
Make `fresh edit` follow symlinks

Closes #73.
Use Perl `abs_path` instead of `readlink` to work with relative paths

`realpath` function from @jasoncodes.

https://github.com/jasoncodes/dotfiles/blob/2ce67436206bac617e24a074b60e5d362ef90872/shell/functions/realpath.sh
Add test for using `fresh edit` to resolve relative links
Include link to the advanced usage section of the wiki
Add fresh_after_build callback

Closes #80.
Remove prompt to update repo when adding fresh lines

Closes #79.
Add a bit of colour to `--help` output
Fix unset variable error when updating with uncommitted changes