Supporting hyphens on endpoints

[gambry]

According with https://developer.wordpress.org/reference/functions/register_post_type/#parameters custom post types can have both underscore and hyphens, resulting in endpoints like my_content or my-content. Current API supports only underscore.

I'm not a WP or a Node developer, but I think this should be either addressed or at least documented.
A temporary workaround is to call the endpoint with site['my-content'].get().

This patch add hyphens supports on routes names replacing.

Ideally the exact regexp should be [^a-z]+, in order to cover any future changes and plugins creating weird endpoints.

[kadamwhite]

@gambry Thank you for the pull request, that's a good catch—I'll merge this and open an issue to discuss the auto-renaming behavior in general