Allow overriding the location of exo plugins by setting and environment variable: EXO_PLUGIN_PATH

Can use : to separate multiple directories to search in.
Merge commit 'f85a1b84eeab444e46e5d5d4901958d8de010a04' into envPluginDir
Merge pull request #12 from tadpol/envPluginDir

Allow for overriding the location of exo plugins via ENV var