Fix for #502.  Make the build script args part of the --fsiargs switch the value of an environemnt variable named fsiargs-buildscriptargs.  This is the best solution to make this restricted set of args explicitly available to the build script.
Add documentation for the new --fsiargs build script arguments approach.
Merge pull request #508 from bentayloruk/fix-fsiargs

Provide env var access to --fsiargs build script args so works on FAKE 3