demo: publish on gh-pages

[christophercr]

I'm submitting a...

[ ] Regression (a behavior that used to work and stopped working in a new release)
[ ] Bug report  
[X] Feature request
[ ] Documentation issue or request
[ ] Support request => Please do not submit support request here, instead see https://github.com/NationalBankBelgium/ngx-form-errors/blob/master/CONTRIBUTING.md#got-a-question-or-problem

Expected behavior

The demo should be deployed to gh-pages to have a nice showcase for our library.

What is the motivation / use case for changing the behavior?

Showcase all the features of our library in order to get the attention from the community so they can start using it.

Environment

Angular version: X.Y.Z
NgxFormErrors version: X.Y.Z