Support for transparent images

[JManch]

Currently transparent pixels just show as black:

This PR replaces transparent pixels with the terminal background:

I've also added an alphathreshold option which I found was useful for refining images with a mix of alpha values. If a pixel's alpha value is below the threshold it won't be visible.

[rashil2000]

Hi, this is a great addition!

Are we sure 50 is a good default value?

[JManch]

It's tough to find a good default value since it depends on the picture. Logically a default of 1 (only hide completely transparent pixels) or 255 (hide all pixels with any transparency) might give more expected default behaviour. However, I've found that both settings tend to give undesirable results in images with a gradient of alpha values. 1 usually results in artifacts from undesired pixels showing and 255 results in too much of the image getting cut off. I figured that 50 is a good middle ground with a bias towards non-transparent pixels so only pixels with an alpha value less than 20% get hidden. It gives good results in the majority of the pictures I've tested with.

[rashil2000]

Hmm, I see. Let's merge this then!