README logo displays poorly with GitHub Dark Mode

[slightknack]

I'm pretty sure that 95% of users on GitHub are using the dark theme at this point. The Passerine logo was designed before this was a thing, and now displays black-text on black. This needs to be fixed:

• We could make the text white, but this is bad for the same reasons
• We could make the background white, but the background would no longer be transparent.
• We could make the background black and the text white, but the background would no longer be transparent
• We could make the text green like the logo; this works because green is between black and white in terms of value, but then the logo has green text.
• We could make the text grey; see the above.
• We could add a white drop-shadow: with light mode this would be invisible, but on dark mode this would outline the text. Likewise, we could just add a white outline to the text.
• I'm not sure if GitHub supports it, but SVGs can be styled with CSS. If GitHub exposes a 'dark-mode' selector, we could change the SVG dynamically to make it work on different platforms. Cross-platform support though.

All in all, a lot of choices. Will have to think about it; what are your thoughts?

[slightknack]

The new logo has a solid background.