Fix labelling envfit plots with a vector of labels

Fixes bug report #74 at github.com: it was documented that you
do not need to use a list(vectors=, factors=) to relabel plotted
items if there is only one type of items (vectors or factors).
Merge pull request #75 from jarioksa/fix-plot-envfit-labels-vector

Fix labelling envfit plots with a vector of labels