Some minor restructuring and clean up

* I prefer lines not to be longer than 80 columns
* In LatexBox_Latexmk: minor restructuring to remove some redundancy
More restructuring: removed redundancy
In LatexBox_Latexmk

Removed much of the redundancy between the cases for
g:Latexbox_latexmk_async.  In particular, the latexmk command should
be the same command in both cases.

The update also include some simple clean up and code improvements.