Add KaTeX support (#164)

* Add render-latex-using-katex.md to Demonstarate how to use the katex

* Update math.html add katex scripts and css

exampleSite/content/posts/render-latex-using-katex.md

@@ -0,0 +1,36 @@
++++
+date = "2019-03-20"
+title = "Render LaTeX using KaTeX"
+description = "Katex support demo"
+katex = "true"
+series = ["Theme", "Hugo"]
++++
+
+Enable katex by adding `katex = "true"` to the [front matter](https://gohugo.io/content-management/front-matter/)  
+
+```toml
++++
+katex = "true"
++++
+```
+
+It's almost a dropin alternative to the mathjax solution,you should just choose one of them.  
+
+Inline math looks like this  
+
+```tex
+This is text with inline math $\sum_{n=1}^{\infty} 2^{-n} = 1$
+```
+
+This is text with inline math $\sum_{n=1}^{\infty} 2^{-n} = 1$  
+and with math blocks:  
+
+```tex
+$$
+\sum_{n=1}^{\infty} 2^{-n} = 1
+$$
+```
+
+$$
+\sum_{n=1}^{\infty} 2^{-n} = 1
+$$