Implement mirror list refresh

[weskoerber]

#18 implemented download mirrors. However, there's no way to update this list -- it is only downloaded when the file at ZVM_HOME/mirrors.json doesn't exist.

A current workaround for this is to just delete the file, and the next time zvm fetches the index, it will redownload the mirror list.

However, this is a hacky solution, and we never want users to manually delete files that zvm depends on, so this should be implemented either as a new command (e.g. zvm mirrors --refresh), or a flag (zvm -r ls).