Features:
When your XP level increases (and under 25 levels with a little bit more XP) the border will increase. It remembers what the highest level you had was.
Setup:
Set the WorldBorder center where ever you like and set the size to 0. Install then the Plugin. The Border start to increase at level 0.
Customizable:
You can choose in the configuration file (or with commands) the Expand message or deactivate it and choose how many blocks the Worldborder will expand per level.
Commands:
/xpborder expandrange <size> Sets the Size
/xpborder expandmessage <message> Sets the Expand-message. null will disable it!
/xpborder xp <count> Sets the last XP expand count
(permission: xpborder.admin)

Note:
The border will not increase when you set or add your levels using the /xp command.

Please let me now in the ratings if there is a bug

Another note:
I've been inactive for a long time but now I've fixed everything and I'm back on track.