Exposing some of `templates.Filename` parameters in `templates.Path` and `as_path()`, `as_filename()`

`templates.Filename` supports configuring how the path is resolved (relative to config, some directory and so on).
I believe it would be nice to have some (or all) of those exposed for `as_path()` and `as_filename()` methods.
I am willing to implement this, just want to make sure this is something that would be accepted :)

Start by inspecting templates.Filename and the as_path() and as_filename() methods to compare which path-resolution parameters are currently available. Clarify which parameters should be exposed and define tests showing the intended behavior before implementing the API change.

Written by the indexing model from the issue text.