completion should not use the logger to output the completion string for the shell

[temsa]

Describe the bug
when I added my custom logger ( with a compatibility layer to behave like winston for caporal ), the completion did not work anymore, because I get the logger prefixes that aren't suitable for a source

To Reproduce
Steps to reproduce the behavior:

1. add a custom logger that prefixes the output with metadata
2. run mycli completion fish, and you'll see the logger custom metadata

Expected behavior
A plain string would be expected so source would work with it

Environment informations (please complete the following information):

• OS: Linux
• OS version: Latest Majaro
• Shell: fish
• Caporal version: 1.3.0

[temsa]

@mattallty this should be pretty simple to fix 😀