Ablity to select a subset of the environmental variable

Currently bens exports all the environmental variables from the yaml file when calling the "environmental" command. If you use the same yaml files for different projects that all require different overlapping subsets of the variables this can make cleaning the environment files you don't need out of the environment difficult. This issues can be worked around with some shell scripting:

eval $(bens environment | grep -E '(var1)|(var3)'

but a built in option could be cleaner:

eval $(bens environement -e var1 -e var2