fix(renderer): allow overwrite server renderer defaults

[katallaxie]

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Build-related changes
• Other, please describe:

This change allows to overwrite the defaults (e.g. isUnaryTag) for the vue-server-renderer. Which should allow to build some more advanced features (e.g. allow to render RSS). Also, it changes the initialization of the SSR renderFunction as it passes along the isUnaryTag to the creation of the compileFunctions.

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• It's submitted to the dev branch for v2.x (or to a previous version branch), not the master branch
• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)
• All tests are passing: https://github.com/vuejs/vue/blob/dev/.github/CONTRIBUTING.md#development-setup
• New/updated tests are included

Other information:

[yyx990803]

This would cause each createRenderer to overwrite the compiler from the previous call. (i.e. creating a second renderer with different options will affect a previously created renderer)

[yyx990803]

I know this is a late review, but I would like to see a more concrete use case example - e.g. how the post-patch API can be used to render RSS feeds.

[yyx990803]

Previous inline comment needs to be resolved

[katallaxie]

Thanks for the late review. I have closed the PR, because it seems to me, that in the end the SSR in is current form is not that well suited to do these things. We are using a fork with some changes to render feeds.