Single quotes in README settings

[sujona]

Thanks for this plug-in, this is a trivial issue! I wanted to add linter options and copied the line
"fortran.linterExtraArgs": ['-Wall'],
from the README into the JSON settings. But you actually need double quotes like
"fortran.linterExtraArgs": ["-Wall"].
Same for "fortran.gfortranExecutable": '/usr/local/bin/gfortran-4.7',.

[pedro-ricardo]

Thank you.
Added to #167