Optimistic loading (new way to display loading state)

[MoOx]

Now, if you remove your PageLoading layouts, you will automatically receive partial data in the correct (future) layout (only head, body won't be defined until isLoading is != true).

Should not be a breaking change if you do still use PageLoading.

If you still use PageLoading, you can define a static property in some layouts to tell Phenomic to send an isLoading props to your specific layout instead of using PageLoading for this specific transition.

You can refer to the minimal change you can do in the docs code if you want to replace PageLoading usage by optimistic loading via isLoading props.

This probably allows you to handle page transition (ref #587) since current layout won't be unmounted.

Closes #836
Closes #824

[MoOx]

@DavidWells does this looks good to you?

[DavidWells]

Looks great! LGTM

[MoOx]

Next time feel free to approve the PR via the dedicated github feature :)

[montogeek]

Oh la la!

[DavidWells]

@MoOx Where is that button? =P I've haven't ever had to use that.