templater: add context.preload(t) to test if the specified template exists
I'm going to remove 'templ' from the resources dict because it is the only
resource that the caller can't provide. This also implies that putting
'templ' into the resources dict creates a reference cycle.
context.preload(t) will be used in place of templater.__contains__().
annotate: pack line content into annotateline object (API)
Just for code readability. We can do that since the annotateline type is
no longer used while computing the history.