tpl: Add truncate template function

tpl: Add truncate template function

This commit adds a truncate template function for safely truncating text without
breaking words. The truncate function is HTML aware, so if the input text is a
template.HTML it will be truncated without leaving broken or unclosed HTML tags.

    {{ "this is a very long text" | truncate 10 " ..." }}
    {{ "With [Markdown](/markdown) inside." | markdownify | truncate 10 }}