Add --coverage option to configure to produce C based coverage reports

This PR is to support the ongoing efforts of the Nodejs testing workgroup under the following issue https://github.com/nodejs/testing/issues/36#issuecomment-255731077

Code coverage has currently been implemented by patching files on the fly in order to add in additional compilation options, but this has obvious limitations should the affected riles change in master.

This initial PR implements a --coverage option for the configure script which will then add the necessary cflags/ldflags in node.gyp in order to build node with C++ code coverage on Linux. Obviously, node builds as per normal without the --coverage option specified in configure.