Allow customising all possible icons

[mohkale]

Hi,

Currently there's quite a lot of icons that cannot be customised in all-the-icons. For example all-the-icons-icon-for-dir hard-codes calls to all-the-icons-octicon "file-submodule" and it's not possible to convert this to use a replacement icon. I've been advising numerous functions to substitute nerd-octicon instead of octicon but that's far from ideal. Would it be possible to move these calls into customizable options that the user can change?