added host config for Stampede @ TACC
config.py: better handling for default initialization of host configs
core.py: search for CUDA toolkit in some pre-defined locations
core.py: handle "extra" args in `build_configure_flags`

This translates '-I...' to 'CPPFLAGS=-I...' and '-L...' to
'LDFLAGS=-L...' when calling `configure`.
core.py: added support for sub-commands in Command