Add parameter set/override from command line

Parameters can now be set or overridden from the command line using the syntax:

x=y

An unlimited number of parameters can be set.  A current limitation is
that a parameter set at the command line may not contain the '=' sign,
as this is used to parse the split between the two parameters.

Some examples:

hit build -v profile.yaml HOST_MATLAB=~/bin/matlab
hit build -v scipy-stack.yaml pyver=3.4 PATH=/usr/local/bin:/usr/bin:/bin
Except that I'm re-raising a more meaningful Exception.  If I don't catch all exceptions here, then the user is going to get an unfriendly stack trace.  I suspect I could catch `ValueError` here, but I'd rather take the risk of catching an exception I shouldn't than dumping if I can give a meaningful message here.