Added math equations and google fonts support

[6cdh]

I added the support of math equations and Google fonts. Both Mathjax and Katex are supported.

To enable Mathjax, edit config.toml to set mathjax = true or only set it in the front matter for a specific post. Katex as well.
Both should not be enabled at the same time.

$$ is used for math block and $ for inline math. For example:

$$
E = mc^2
$$

or $E=mc^2$.

There is no cost if disabled.

Google fonts can be used for custom font-family. For example, edit config.toml:

[params.assets]
googleFonts = ["Quicksand", "Open Sans"]
customCSS = ["css/custom.css"]

/static/css/custom.css:

body,
button,
input,
select,
textarea {
  font-family: Quicksand, "Open Sans", Helvetica, Tahoma, Arial, STXihei, sans-serif;
}

There is no cost if googleFonts is an empty list.