Added support for passing function handles as columns formatters

[emilhe]

The purpose of this pull request is to demonstrate how function handles can be passed from Dash to JavaScript. The only required change to the source code (apart from importing dash-extensions) is the following lines,

for(let i=0; i < columns.length; i++){
    columns[i] = resolveProps(columns[i], ["formatter"])
} 

which converts data passed from Dash into the desired function handle. Now, if you define the function in a JavaScript asset (i.e. in a *.js file in the assets folder),

window.myNamespace = Object.assign({}, window.myNamespace, {
    tabulator: {
        printIcon: function (cell, formatterParams, onRendered) {
            return "<i class='fa fa-print'></i>";
        }
    }
});

it can be passed to the component with the following syntax,

from dash_extensions.javascript import Namespace
...
ns = Namespace("myNamespace", "tabulator")
...
columns = [{"formatter": ns("printIcon")}, ...]

which yields a table with the custom format as intended,

Here is the complete python code,

import dash_tabulator
import dash
import dash_html_components as html
from dash_extensions.javascript import Namespace

# The namespace here must match the name space of the JavaScript asset.
ns = Namespace("myNamespace", "tabulator")
# Setup some data
columns = [
    {"title": "Name", "field": "name", "width": 150, "headerFilter": True, "editor": "input"},
    {"title": "Print", "field": "print", "hozAlign": "center", "formatter": ns("printIcon")}
]
data = [
    {"id": 1, "name": "Oli Bob", "print": "stuff1"},
    {"id": 2, "name": "Mary May", "print": "stuff2"},
]
# Create example app.
external_stylesheets = ['https://stackpath.bootstrapcdn.com/bootstrap/4.1.3/css/bootstrap.min.css',
                        'https://cdnjs.cloudflare.com/ajax/libs/font-awesome/5.15.1/css/all.min.css']
app = dash.Dash(__name__, external_stylesheets=external_stylesheets)
app.layout = html.Div([
    dash_tabulator.DashTabulator(id='tabulator', columns=columns, data=data),
])

if __name__ == '__main__':
    app.run_server(debug=False, port=7777)

[pjaol]

Looks good, missing the requirements.txt update, will update here
Thank you this is a fantastic contribution!