Add capability for mathjax equations

[DanOvando]

Description

Allows users to type equations in blog posts using Mathjax

Added footer_mathjax.html and incorporated footer_mathjax.html into footer.html, per instructions provided here

Motivation and Context

Closes #124

How Has This Been Tested?

Pushed changes, and built new site using blogdown::build_site and the mathjax modified theme. Added equations to both .md and .Rmd files, build website, and verified that equations are indeed visible

Hugo Version:
0.36
Browser(s):
Firefox, Safari, Chrome

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation, including theme.toml, accordingly.
• I have read the Contributing Document.