fix(task): load config on task execution

This fixes the problem that the tasks configuration may not be
initialized when the plug-in is loaded.

This is the case if a user executes

grunt.loadTask('grunt-dgeni') // or
require('load-grunt-tasks')(grunt);

prior to configuring the grunt targets.

Closes #1
Merge pull request #2 from Nikku/load-fix

fix(task): load config on task execution