[feature] Line whitespace control

This PR introduces line whitespace control character `~` which can be used the same way standard whitespace control character `-` can be, for example:

* `{{~ "Trim to my left, but only until first line break" }}`
* `{%~ for word in ["Trim to my left, but only until first line break" %}`

I'm not sure if trimming on the right side is possible.

* `{% for word in ["Trim to my right, but only until first line break" ~%}` does not stop at line break :(

Fixes issue #1423