Add a build option for transparent background

[gpollo]

This PR allows the user to build the application with background transparency using make transparent=1.

[clbr]

Ncurses docs state that to use color -1, you must first call use_default_colors. I also think that this doesn't need to be an option at all, but replacing the BLACK color with -1 should work in most terminals.

[gpollo]

use_default_colors is being called here. And yeah, I don't really if it's a good thing to default to -1 tho.

[gpollo]

After some testing, always enabling transparency works fine under TTY, but I believe it is a matter of taste whether one wants transparency (it just looked great with my background under X desktop).

[clbr]

None of my terminals use compositing, and under all of them -1 is just the default bg color (black for all but xterm, which is white). So please make it the default.