Prefer stow over hand-rolled solution

After discovering GNU stow on Nix IRC, refactored install and setup
scripts to consume it.

Code is vastly simplified as a result.

configs/shared/.gitconfig

@@ -1,21 +0,0 @@
-[user]
-	name = William Carroll
-	email = wpcarro@gmail.com
-[alias]
-	recent = for-each-ref --count=10 --sort=-committerdate refs/heads/ --format=\"%(refname:short)\"
-	today = ! git log --date=relative --since=00:00:00 --all --no-merges --oneline --author=\"$(git config --get user.email)\"
-	changed-files = ! git --no-pager diff --name-only $(current_branch) $(git merge-base $(current_branch) master)
-	conflicts = ! git --no-pager diff --name-only --diff-filter=U
-	yday = ! git log --name-only --since=yesterday.midnight --until=today.midnight --author=\"$(git config --get user.email)\"
-	patch-grep = log -p -S
-[credential]
-	helper = osxkeychain
-[push]
-	default = current
-[rebase]
-	autosquash = true
-	autostash = true
-[github]
-	user = wpcarro
-[core]
-	excludesfile = ~/.gitignore