Define highlight colors depending on the secondary color

[jumjamjohn]

Hi, thank you for your work! I'm really enjoying this plugin and I'd be glad if I can help to make it better 😃

Issue

When I do ls | grep something and also when vim shows an alert message, output text is not readable since the font color is the same as the background.

macOS hyper version: 2.0.0 hyper-pokemon version: 0.4.3

What I did

I added constants highlight and secondHighlight, and set them to lightWhite and white respectively. The constants are defined depending on the value of secondary. When secondary is a dark color, highlight colors will be white, otherwise black. I chose #000000 and #FFFFFF for lightWhite, and #686868 and #C7C7C7 for white because those are default colors in .hyper.js and easy to recognize. I've checked almost all themes.

[klaudiosinani]

Looks nice! Thank you a lot taking the time to submit the PR : )