Linked and formatted user names

Implements an event which can modify the link below usernames, and the
displayed user name.

When no name supplied, the name of currently logged-in user is used.

Fixes: 
 * [FS#1558](https://bugs.dokuwiki.org/index.php?do=details&task_id=1558) - Link to user page in function tpl_userinfo
 * [FS#2151](https://bugs.dokuwiki.org/index.php?do=details&task_id=2151) - more options for showuseras -- actually the suggested fix is using interwiki links...

2 question left:
 * [x] Howto combine current behavior and a new option for own userlink
 * [x] Howto easily configure the userprofile link?

Implemented as:
 * Add 4th 'showuseras' select option: 'username with interwikilink'
 * FS#2151 interwiki links for configurable links 
 * [FS#2713](https://bugs.dokuwiki.org/index.php?do=details&task_id=2713) 'use existing internal link color for internal interwiki links' is implemented for interwiki links as local wiki links.


* [x] check usage of `<bdi>` tags