Support generating for only some languages

[utensil]

As described in the modified README:

If you wish to generate and compile for only some of these languages or algebras, run something like

make GEN_LANG="csharp cpp python rust" c hyperbolic dual r2 r3 mink quat r111 spacetime cga pga3d

with languages or algebras you don't need removed from the command parameters.

For example, if you only want pga3d for rust, just run:

make GEN_LANG="rust" pga3d

Additionally, it also ensures that the files generated for each language fall into separate directories per language and the corresponding directories will be created automatically.

RFC.