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
Ok, I guess that's fine then.