Externalise wiki file settings to ftplugin file

There was a wiki.vim in ftplugin already with half the settings for
line-wrapping, I just moved the other command into it from the vimrc as
well.
Sets buffers to delete when editing TeX files

In vimrc buffers are set to hide instead of delete when closing with the
command `set hidden`. This causes problems with latex-suite's error log
and quick-fix list, which seem to assume that the previous list/log
buffer has been deleted before it shows the next one, which causes
errors. Turning off "hidden" for .tex files solves this.
Set AcroRead as PDF reader for tex files on Windows

The line setting Acrobat Reader to read PDFs which I wrote on Windows
had been commented out since I used this vimrc on other machines. It has
been moved to a conditional statement in the ftplugin directory to only
set it:
 * if we are opening a .tex file
 * if we are running some form of MS Windows™