Change timing of completion to enable to configure its error message

[sttk]

This pr moves completion into execute method, because the current timing is before loading config files.

This modification is needed for gulp-cli#161.

[phated]

I believe this is breaking, since code might have assumed that execute was never run when completions were called, but otherwise looks good! I'll merge and update the commit message.

[sttk]

@phated Thank you for merging!