Add name and value props to Button

[tcbegley]

As requested in #553

Example usage:

import dash
import dash_bootstrap_components as dbc
from flask import request

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

app.layout = dbc.Container(
    dbc.Form(
        [
            dbc.Input(name="input"),
            dbc.Button("Submit", name="submit", value="submitting"),
            dbc.Button("Submit alt", name="submit-alt", value="submitting-alt")
        ],
        method="POST",
        action="/test",
        id="form",
        prevent_default_on_submit=False,
    ),
    className="p-5",
)


@app.server.route("/test", methods=["POST"])
def test():
    return request.form


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