v2.2.0

Add support of 2 themes: Solarized (dark), and Solarized-light

In these 2 color schemes, I follow the Solarized guideline. For <em> and <strong> syntax, I use the color similar to normal text. I aim to pick different color in other themes because of display problem in some fonts or some Windows system. This is still a test to see your comments.

Happy 2019