Support post loaders for template blocks

[davidguilherme]

I know there is already a MR that sorts this issue out but I think bringing the postLoader option back is not the best approach to solve this issue. This merge request proposes a solution that allows post loaders to be applied after the template loader with no extra options.

Motivation
In our project we would like to be able to execute a custom babel-macro in our templates and that requires us to process the compiled template using babel. Currently the v15 does not allow this. See: #1309.

Description
The solution implemented in this merge request detects which loaders have enforce: 'post' option looking at the pitchExecuted flag in the loader object. Once the VueLoaderPlugin injects the pitcher at the beginning of the loaders list that means the pitcher's pitch would be executed first than any loader. So, every loader which has pitchExecuted equals true would be a post loader. In that way we can inject the template loader between the post loaders and the rest of them.

Fixes #1309.

[yyx990803]

Thanks! This is great. Didn't think that you use pitchExecuted to detect them.