Allow override of the index template

[Benoss]

This small change allow to quickly override the Jinja template by proving your own base template.
If you don't set app.config["CLICK_WEB_MAIN_TEMPLATE" there is no changes to the current behaviour of the blueprint

Example folder structure:

main.py (contain a click app called cli)
webcli.py (the file bellow)
webcli/static/custom.css (the css override for the app)
webcli/templates/my_main_clickweb.html.j2 (A modified version of template show_tree.html.j2)

from click_web import create_click_web_app
from flask import Blueprint
import main

app = create_click_web_app(main, main.cli)
# Expect any custom static and template folder to be located in same folder as this script
# Make custom folder reachable by "custom/static" url path and add templates folder
custom_folder_blueprint = Blueprint(
    "custom", __name__, static_url_path="/static", static_folder="webcli/static", template_folder="webcli/templates"
)
app.register_blueprint(custom_folder_blueprint)

# Set CUSTOM_CSS in flask config so click-web will use it.
app.config["CUSTOM_CSS"] = "custom.css"
app.config["EXPLAIN_TEMPLATE_LOADING"] = True
# Override the main template to one in webcli/templates/my_main_clickweb.html.j2 directory
app.config["CLICK_WEB_MAIN_TEMPLATE"] = "my_main_clickweb.html.j2"`  

if __name__ == "__main__":
    app.run(debug=True, port=5000)

[fredrik-corneliusson]

Nice, would it be possible to add an example for this like example/custom/