feat(git): add purge alias

The combination of -d and -X is especially convenient for removing
`node_modules` from mono repositories.

Basically -f is always needed to make git clean do anyting useful.

.config/git/config

@@ -8,6 +8,7 @@
 	difft = -c diff.external=difft diff
 	fixup = commit --fixup=HEAD
 	please = push --force-with-lease
+	purge = clean -dfX
 	remotes = remote --verbose
 	root = rev-parse --show-toplevel
 	whoami = config --get user.email