user filter which takes precedence over htmlize-buffer if set

Added the following interactive functions: 

`imp-set-user-filter` and `imp-remove-user-filter`

This allows the user to define a custom html-producing function on the current buffer. When set, takes precedence over the htmlize feature and regular html files. When removed, it's back to the previous behavior (no processing for html buffers, `htmlize-buffer` for non-html buffers).