Multi-line formula RENDERING bug.

[H-Lumos]

Right markdown syntax used, wrong rendering output.

## this is a level 2 title

now, let's test formula feature, Maxwell's Equations are shown below:

$$
\nabla \cdot \vec B = 0 \\
\nabla \times \vec B = - \frac{\partial \vec E}{\partial t}
$$

Side preview on VSCode:

After moffee live .md, wrong output:

[BMPixel]

Thank you. I'll look into it. Maybe it's about configs for MathJax.

[BMPixel]

MathJax v3, which is used in moffee, does not support double backslash for new lines. Since MathJax is no longer mantained, I change to Ketax in this commit. It should be working now. Before next release, to apply the changes, please install moffee from the repo.