template: Preprocess parameters before htmlizing.

template: Preprocess parameters before htmlizing.

Consider a template like:

[[!template type=note text="""
[[!inline pages="*foo*"]]
"""]]

The text parameter is htmlized before being passed into the template (in
case the template wraps it in a <span> that prevents markdown from
htmlizing it later).

But, when markdown sees "*foo*", it turns that into <em>foo</em>.
Later, when preprocessing the inline directive, that leads to suprising
results.

To fix this, I made template parameters be preprocessed (and filtered)
before being htmlized.

Note that I left in the preprocessing (and filtering) of the template
output at the end. That's still relevant when the template itself contains
preprocessor directives.