Document --source-only and add --push-only

The `--source-only` already works to only build and not deploy. But
sometimes we want to just push and not build. An example of that is
when you want to use Docker to build the project, avoiding the
installation of the whole Ruby environment in your computer.

This commit adds `--push-only` to add the ability to execute only the
actions related to `git` and avoid the project build.

No API was broken by this commit, it's a minor change.

deploy.sh

@@ -15,9 +15,14 @@ Options:
                            deploy branch.
   -n, --no-hash            Don't append the source commit's hash to the deploy
                            commit's message.
+      --source-only        Only build but not push
+      --push-only          Only push but not build
 "
 
-bundle exec middleman build --clean
+
+run_build() {
+  bundle exec middleman build --clean
+}
 
 parse_args() {
   # Set args from a local environment file.
@@ -200,4 +205,11 @@ sanitize() {
   "$@" 2> >(filter 1>&2) | filter
 }
 
-[[ $1 = --source-only ]] || main "$@"
+if [[ $1 = --source-only ]]; then
+  run_build
+elif [[ $1 = --push-only ]]; then
+  main "$@"
+else
+  run_build
+  main "$@"
+fi