latexmk '-lualatex' option support

It seem like `latexmk` does not work correctly when '-lualatex' and
'-e "$latex=...."' options were passed together.

This commit will fix this issue by ignoreing -e options when '-lualatex'
is specified in `g:LatexBox_latexmk_options`

I'm not really familiar with `latexmk` but the old style (?) lualatex
specification ('-pdflatex="lualatex %O %S"') is not supported with
this commit.