Add download prop to Button

[tcbegley]

Add download prop to Button as requested in #526

[tcbegley]

Here's an example. With the following directory structure

.
├── app.py
└── file.txt

and the following contents in app.py.

# app.py
from pathlib import Path

import dash
import dash_bootstrap_components as dbc
import flask

HERE = Path(__file__).parent

app = dash.Dash(external_stylesheets=[dbc.themes.BOOTSTRAP])

app.layout = dbc.Container(
    dbc.Button(
        "Download",
        href="/file.txt",
        download="test.txt",
        external_link=True,
    ),
    className="p-5",
)


@app.server.route("/file.txt", methods=["GET"])
def send_file():
    return flask.send_from_directory(HERE, "file.txt")


if __name__ == "__main__":
    app.run_server(debug=True)

text file can be anything, just to test downloads.