Adding Latex math mode support using jektex

[yagarea]

Hi, this is simple PR that adds Latex rendering support using jektex.

In current state it

• renders latex display and inline math modes
• does not add any JS to page it self.

While adding this feature I have found out. That jekyll-seo-tag does not escape string and can create invalid html head. I have opened issue on its repository.

Tell me what do you think about this PR. Currently this is the minimal jektex setup. I have few suggestions that you may be interested in:

• adding katex css and fonts to make them local resource
• ditching seo tag and
  • implement it in liquid directly in head
  • using another plugin that actually works