"Higher order" environment variable in enable script

[pinkwah]

See scheme in https://github.com/equinor/equilibrium/pull/37

Instead of providing multiple hardcoded environment variables, we can provide a single environment variable _PRE_KOMODO, that lists all of the environment variables that have been modified. Eg, _PRE_KOMODO=PATH:MANPATH tells a deactivate script that Komodo has been enabled previously and that PATH and MANPATH environment variables have been modified in some way.

Then, if PATH is set, it should be unset. If _PRE_KOMODO_PATH is set, export PATH=$_PRE_KOMODO_PATH and unset it.

This gives us the flexibility of changing which environment variables are set in the future, plus removes the possibility for future bugs such as #157 .