Add a blank space after the last bullet in 'tree' format

[alex-mashin]

This PR is made in reference to: #672

Tree format needs a blank space after the bullet. Expected:

• node1 ** node11

Actual:

*node1 **node11

Reason: after this change, {{#ask:...|format=tree}} can be safely wrapped with

{{#tag:plantuml| @startmindmap {{#ask:...|format=tree}} @endmindmap }}

as is.

if External Data or some other extension provides the tag .

Now the output of {{#ask:...|format=tree}} has to be processed with a regular expression to inject the needed spaces.

Example: https://traditio.wiki/Template:MindMap.

Fixes #672