Add addon_type prop to DropdownMenu

[pbugnion]

Passing the addon_type prop to DropdownMenu allows embedding dropdowns in input groups. For instance:

See below for the code used to generate that app. The value in the Input is generated dynamically as the dropdown changes.

This PR also adds unit tests for DropdownMenu.

This resolves issue #52 and allows me to finish PR #51 .

---

Test code:

import dash
import dash_bootstrap_components as dbc
import dash_html_components as html
import dash_core_components as dcc
from dash.dependencies import Input, Output

app = dash.Dash(
    __name__, 
    external_stylesheets=['https://stackpath.bootstrapcdn.com/bootstrap/4.1.3/css/bootstrap.min.css']
)

app.layout = dbc.Container([
    html.H2('Dropdown add on type demonstration'),
    dbc.InputGroup(
        [
            dbc.DropdownMenu(
                [
                    dbc.DropdownMenuItem("hello", id="demo-input-hello"),
                    dbc.DropdownMenuItem("world", id="demo-input-world")
                ],
                addon_type="prepend",
                label="Label"
            ),
            dbc.Input(placeholder='username', id="demo-output")
        ]
    )
])


@app.callback(
    Output("demo-output", "value"),
    [Input("demo-input-hello", "n_clicks_timestamp"), Input("demo-input-world", "n_clicks_timestamp")]
)
def _(t_click_hello, t_click_world):
    if not t_click_hello and not t_click_world:
        return ''
    elif t_click_hello is None:
        return 'world'
    elif t_click_world is None:
        return 'hello'
    elif t_click_hello <= t_click_world:
        return 'world'
    else:
        return 'hello'
    
    
if __name__ == '__main__':
    app.run_server(port=8888, debug=True)

[tcbegley]

Looks great!