feat(compile): post-t'clusion linking now optional

Can be controlled by returning an object with `{alreadyLinked: true}`
from $transclude.

Can you come up with a better name than `alreadyLinked`?

Related to #7270: While not a complete (automatic) solution to that issue, with this new feature the bug can at least be avoided.

Also, this is meant to be an advanced feature for those who want to do some more ninja-esqe stuff with transclusion. It is not meant to be used regularly or by novices who are new to $compile.