Add an extra install option to disable the compilation step

[goanpeca]

@pistolero thanks for the package :-)

I am building conda packages for libsass (the C library and the CLI) and for libsass-python. Since with conda we dont need to include all the extra compilation steps (as conda will handle libsass standalone) would you be ok with adding an option for install so that it does not perform the compile steps? Something that would yield something like:

python setup.py install-no-compile

or

python setup.py install --no-compile

I can make the PR