Make cursor visible on dark background themes

[gdubicki]

The default black cursor is hardly visible on dark background themes such as "solarized dark". Preffered solution would be using white cursor for dark backgrounds and black for light, but also good would be to use a single white & black that is visible on both, such as https://github.com/gnachman/iTerm2/blob/master/images/IBarCursor%402x.png .