Refactor LaTeX toggle and remove global config var

The toggling of LaTeX rendering was being facilitated by a global config
variable. This added an unnecessary layer to the command chain to toggle
LaTeX rendering. The global config var was also exposed in the user
settings, which was never an intention.

The refactored code removes the global config var and
MarkdownPreviewView's now directly observe the
'markdown-preview:toggle-render-latex' command.
Refactor LaTeX rendering and rm global config var

Rendering LaTeX in a preview window is now exposed through a boolean
argument of renderer.toHTML. The associated global config var has been
removed, and mathjax-helper no longer conditions on weather it should or
shouldn't render LaTeX; if renderer.toHTML receives a false to render
LaTeX then it simply runs different branches of execusion that never
call mathjax-helper methods.