Support the new target: "static"

[francois2metz]

The target "static" has been introduced in nuxt 1.13. It would be nice if prismic-module would support it.

Right now, using the static target, there is one call to the prismic api in the generated site:
https://github.com/nuxt-community/prismic-module/blob/c824747c3e90634324236d897097da11e7a2d831/src/templates/plugins/prismic.js#L19

[lihbr]

Closing in favor of #134

tl;dr;

We'll consider switching to the .client method soon to init the API which comes with the benefit of only initing the API when actually called. We just need to assess the impact that could have on existing users before!