Add option to ignore missing source files

This is useful when using dynamic fresh lines:

    fresh zsh/$PROFILE --file=~/.zshrc --ignore-missing

Fixes #88.
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
Merge pull request #89 from freshshell/fresh-edit-follow

Make `fresh edit` follow symlinks
Include link to the advanced usage section of the wiki
Merge pull request #90 from freshshell/update-readme

Update README
Add fresh_after_build callback

Closes #80.
Merge pull request #92 from freshshell/after-build

Add fresh_after_build callback
Remove prompt to update repo when adding fresh lines

Closes #79.
Merge pull request #91 from freshshell/fresh-add-tweaks

Remove prompt to update repo when adding fresh lines
Merge pull request #93 from freshshell/nounset

Treat unset variables as an error when performing parameter expansion
Add a bit of colour to `--help` output
Fix unset variable error when updating with uncommitted changes
Allow FRESH_LOCAL to be overridden during installation
Add table of contents to the README

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Fix paths in .freshrc example