Fix editing list members using edit-cache

This is a fix for something I already fixed but apparently got lost in some of the rebases. The list of parameters was not properly taken when the members should be str.

Before:

./cerbero-uninstalled -t edit-cache gstreamer --steps fetch extract configure compile
steps: [['f', 'e', 't', 'c', 'h'], ['e', 'x', 't', 'r', 'a', 'c', 't'], ['c', 'o', 'n', 'f', 'i', 'g', 'u', 'r', 'e'], ['c', 'o', 'm', 'p', 'i', 'l', 'e']]

Now:

steps: ['fetch,', 'extract,', 'configure,', 'compile']

Additionally, I have added that you can easily empty lists by not setting any extra argument. e.g:

./cerbero-uninstalled -t edit-cache gstreamer --steps

This will effectively clean the steps list without resetting the whole recipe