Add support for displaying vaxe's current build file.

Vaxe is a plugin for haxe, which is a statically typed language.

http://haxe.org/
https://github.com/jdonaldson/vaxe

Haxe uses a build file similar to a make file, but this file does not have a reliable default name.  The vaxe plugin can show the current build in a status line, which is helpful.  This pull request adds this information to the powerline bar, just before the current haxe file path.