Set flickity as peerDependecy

[emateu]

Scenario

Combining flickity with multiples plugins, like flickity-fullscreen and flickity-as-nav-for.

Issue

Since we have flickity as a dependency, will extend flickity from it's own node_modules instead of extending the one of the project's node_modules.

This makes impossible to combine two plugins.

Solution

Define on the plugin, flickity as a peerDependency.

--

If you merge this, we should do the same on https://github.com/metafizzy/flickity-as-nav-for