Support ln=target in LS_COLORS (#774)

[LarsHaalck]

It is allowed to set ln=target instead of a color (e.g. like this), which was not supported by exa.

Setting ln=target highlights a link in the same color as the referred file. Dangling/orphaned links are always colored using the specified or= color.

See related issue (#774) for details

[jbrubake]

When I built exa using this patch, it does properly color symlinks, but for some reason certain combinations of terminal size (width I think, not height) and directory contents cause exa to always use the --oneline listing style.

I can't figure out what the exact situation is and looking at the patch, I can't see why it would have that effect, but it does @LarsHaalck

[LarsHaalck]

You are right, but I think this is an issue with a previous commit and not with this PR. I also have this when building the exa from source without my PR.

I think it is related to this #990

[jbrubake]

I didn't see how it could be related to your code. I should have checked to see if their were commits between the latest release and HEAD. I'll take a look later.

Nice work on the symlink patch. Since I found that feature of ls I find not having it annoying