support for view and module loading by start and run commands

[bcumming]

This PR implements support for loading views and making modules available inside an environment via command line arguments to start and run commands.

# start with the view named develop loaded
uenv start --view=develop myenv

# start with modules available
uenv start --modules myenv

# run a command with modules available
uenv run --modules myenv -- bash -c "module load cray-mpich; mpicc test.c"

# run a command with a view loaded
uenv run --view=develop myenv -- mpicc test.c

• add lib/envvars.py that implements classes that support setting and modifying environment variables.
  • support for scalar variables that take a single value and list variables (e.g. PATH) that are assembled by setting, appending and prepending paths.
  • support for accumulating operations on collections environment variables
  • support
• instead of sourcing activate.sh when loading a view, the script is parsed and export commands is generated based its contents
  • this allows us to perform operations like overriding or preserving existing list variables
  • in the future this can be used to generate information required to deactivate a view
• changed the behavior of uenv view to preserve existing values of variables like PATH, LD_LIBRARY_PATH, etc
• add unit tests for the new environment variable code

[simonpintarelli]

lgtm