Add support for :usermenu hook

[giterlizzi]

Hi!
I have added in develop branch a new hook called :usermenu. This hook permit to add new items in user menu using a standard DokuWiki page.

This is a sample DokuWiki page with "three" sections (using "heading" syntax) and using horizontal ruler for create a "divider". It's also possible add a RSS feed to add automatically items in menu (only links) and ~~NOCACHE~~ to highlight the current link.

== News ==
  * {{fa>plus?text-success}} [[:news:add|Add news]]
  * {{fa>newspaper-o}} [[:news:start|All news]]

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== Latest Linux releases ==
{{rss>https://www.kernel.org/feeds/kdist.xml 5 1h }}

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== Wiki ==
  * [[wiki:info|]]

~~NOCACHE~~

NOTE It's possible to create a global hook and "user-specific" hook (eg. :user:<my-username>:usermenu)

Joseph