Add option to make the quickfix window not active when shown

One can now do
`let g:LatexBox_quickfix_active=0` to make the quickfix window not
active when shown: the cursor stays in the current window.
If `g:LatexBox_quickfix` is set to `0`, this setting has no effect.

This is useful for me because I call Latexmk automatically when the buffer is written, and this allows me to see errors while not interrupting the flow of editing.