Add \LaTeX function

Add function that produces the `\LaTeX` symbol. Only thing different from `\KaTeX` symbol is the `A`.

Values tweaked to most closely represent this LaTeX symbol from Wikimedia: ![Wikimedia LaTeX symbol](https://upload.wikimedia.org/wikipedia/commons/9/92/LaTeX_logo.svg)

Screenshot:

![latex symbol](https://cloud.githubusercontent.com/assets/4154884/4431270/5618f6dc-465f-11e4-910c-d5102701bcfc.png)