Implement support for [codeblock] tag in help

Created by: akien-mga

It allows to define a multiline space-indented code block that will be properly parsed by the reST converter for the online docs. The in-editor help understand the [codeblock] tag as it supposedly understand [code] already (i.e. it just seems to discard it, though the code was supposed to switch it to a monospace font, but that's likely another issue.