Case insensitive extension matching for icons & colors

[desbma]

This changes the extension matching algorithm for icons & color selection to force lowercase search, effectively making it case insensitive.

I noticed that the icons & coloring where different for .jpg and .JPG files. For the icons I can generate a new ~/.config/lf/icons file with all extensions lowercase and uppercase but that is not ideal since it does not cover mixed case for example Jpg extension. For colors I use vivid to generate the LS_COLORS environement varibale, wich only generates lowercase extensions.

This change makes lf behavior in line with other programs I checked (I suspect they are many more):

• GNU ls handling of LS_COLORS
• lsd handling of LS_COLORS and icons
• vivid

[lahwaacz]

There is #805 already which does it only for icons.

[desbma]

That's right, sorry I missed that.
I'm keeping this PR open though, because it makes sense to do the same for colors.

[gokcehan]

@desbma Sorry for late reply and thanks for the patch.