Add Blueprint level cli command registration

Implements #1357.
Adds ability to register click cli commands onto blueprint.

tests/test_cli.py

@@ -23,7 +23,7 @@ import pytest
 from _pytest.monkeypatch import notset
 from click.testing import CliRunner
 
-from flask import Flask, current_app
+from flask import Flask, current_app, Blueprint
 from flask.cli import (
     AppGroup,
     FlaskGroup,
@@ -609,3 +609,46 @@ def test_run_cert_import(monkeypatch):
     # no --key with SSLContext
     with pytest.raises(click.BadParameter):
         run_command.make_context("run", ["--cert", "ssl_context", "--key", __file__])
+
+
+def test_cli_blueprints(app):
+    """Test blueprint commands register correctly to the application"""
+    custom = Blueprint("custom", __name__, cli_group="customized")
+    nested = Blueprint("nested", __name__)
+    merged = Blueprint("merged", __name__, cli_group=None)
+    late = Blueprint("late", __name__)
+
+    @custom.cli.command("custom")
+    def custom_command():
+        click.echo("custom_result")
+
+    @nested.cli.command("nested")
+    def nested_command():
+        click.echo("nested_result")
+
+    @merged.cli.command("merged")
+    def merged_command():
+        click.echo("merged_result")
+
+    @late.cli.command("late")
+    def late_command():
+        click.echo("late_result")
+
+    app.register_blueprint(custom)
+    app.register_blueprint(nested)
+    app.register_blueprint(merged)
+    app.register_blueprint(late, cli_group="late_registration")
+
+    app_runner = app.test_cli_runner()
+
+    result = app_runner.invoke(args=["customized", "custom"])
+    assert "custom_result" in result.output
+
+    result = app_runner.invoke(args=["nested", "nested"])
+    assert "nested_result" in result.output
+
+    result = app_runner.invoke(args=["merged"])
+    assert "merged_result" in result.output
+
+    result = app_runner.invoke(args=["late_registration", "late"])
+    assert "late_result" in result.output