#1. added platform detection for win32 as execFile will only run an 'executable' and 'npm' is NOT a binary on win32, its actually a .cmd file. Also passed in currently executing environment, as certain platforms  may contain variables that are used by invoked toolchains.
#2. added ability to take npm flags from primus's package.json using 'npm_install_flags_<PLATFORM>', such that npm use can be tweaked - intention here is that a primus-live package 'consumer' can update this parameter in package.json for the platform in use.