Use ANSI clear for fully transparent pixels

[kj800x]

It would be nice for fully transparent pixels to print out blank, so that they can use the terminal's default background color. I figure this can be done with ANSI reset/clear?

For example, when I try to use termpix to print out this png with a transparent background, I instead get a weird dark grey background (which is a slightly different grey than my terminal background).

(Note that I'm using mmacedoeu's fork which seems mostly unchanged from this repo, just with the build issues fixed.)

Raw Image:

Rendered in Preview Application:

Rendered by Termpix:

[kj800x]

Related to #14, although the proposed solution there seems to be to have it render out a checkerboard pattern in the background, which is not what I'd expect/want. At the very least, I think there should be a flag to toggle between checkerboard/truly transparent rendering of transparent pixels.