Add conditionally included submodules

Came up with this in #25

Collect common rules into a separate _footer which is conditionally included based on the value of a variable in the task-specific makefile. In `common/Makefile` we could have something like:

PROFILE_RAW_DATA := FALSE
PROFILE_FEAUTRES := TRUE

Downstream, we could have stuff like:

REGRESSION_TASK := FALSE
CLASSIFICATION_TASK := TRUE
PERMUTATION_OK := TRUE

to turn collections of evaluation metrics on or off.