Adding option to load external layouts

[xhluca]

About

This PR will add the option to add external layouts (5 popular layouts). The user will need to do:

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

import dash_cytoscape as cyto

# Load extra layouts
cyto.load_extra_layouts()

app = dash.Dash(__name__)
server = app.server
...

In order to load the layouts. This function will load the appropriate bundles (dash_cytoscape_extra.[min|dev].js). Here's one of the new layouts:

The app is saved as demos/usage-elements-extra.py.

Pre-Merge checklist

• The project was correctly built with npm run build:all.
• If there was any conflict, it was solved correctly
• All changes were documented in CHANGELOG.md.
• All tests on CircleCI have passed.
• All Percy visual changes have been approved.
• Two people have 💃'd the pull request. You can be one of these people if you are a Dash Cytoscape core contributor.

Reference Issues

Closes #32

Other comments

[chriddyp]

💃 very nice!