Allow spaces in matched square bracket tokens

GetPot was breaking "[tokens like this]" into "[tokens" "like"
"this]", which was fine for section names, but now in the near future
we can use square brackets for preprocessor-like directives too.
Merge branch 'master' of ssh://github.com/libMesh/libmesh
Merge branch 'master' of ssh://github.com/libMesh/libmesh