Jinja: Forbid stripping newlines

This creates a bit prettier theme output. When newlines from ends are
stripped, constructions like
```
<header>
{% include "header.html" %}
</header>
```
end compiled with the ending tag at last line of the include

I think it is a bit shame that Pelican does not have any method to
configure Jinja from the theme itself, since this is more theme-related
than content-related.

pelicanconf.py

@@ -13,6 +13,14 @@ RELATIVE_URLS = True
 
 PATH = 'content'
 THEME = 'test-theme'
+JINJA_ENVIRONMENT = {
+	# The default:
+	'trim_blocks': True,
+	'lstrip_blocks': True,
+
+	# Custom
+	'keep_trailing_newline': True,
+	}
 
 TIMEZONE = 'Europe/Prague'