feat: `LS_ICONS` for icon overrides

[pagerc]

This is a continuation of the unaddressed exa issue: https://github.com/ogham/exa/issues/559

LS_ICONS would make a good corollary to LS_COLORS for overriding eza's internal file extension to icon mappings. Example derived from (https://github.com/ogham/exa/issues/559#issuecomment-517953531):

LS_ICONS="*.rb=e21e:*.vim=e62b:*.sh=f292:"

[cafkafk]

We'd love to see a PR for this if anyone has time :D

[gierens]

Since I was just in that part of the code the other day I thought I give it a shot, see the linked PR draft :D

[gierens]

I ported my draft from #909 to #840 ... so overriding icons will be a feature of the config file once that is merged.

[fgclue]

if this gets added i'm going to finally switch that folder icon with the better one