Add git configurations for submodules

Add git configurations for better work with submodules taken from
https://medium.com/@porteneuve/mastering-git-submodules-34c65e940407.

Add alias that syncs submodules when pulling.
Include information of changes in submodules in diff and status.

.config/git/config

@@ -16,3 +16,8 @@
 	excludesfile = ~/.config/git/ignore
 [alias]
 	root = rev-parse --show-toplevel
+	spull = "__git_spull() { git pull \"$@\" && git submodule sync --recursive && git submodule update --init --recursive; }; __git_spull"
+[status]
+	submodulesummary = true
+[diff]
+	submodule = log