Fix nodeExecutable support

Currently, the nodeExecutable variable from the pom is ignored. The plugin will only work if you have node installed natively on the system. With these changes, users will be able to specify the directory of the node executable. This will enable support for nodejs-maven-plugin, or if they have the node executable in the stack somewhere. 

Please note: to use the native node, you will now have to pass "node" to nodeExecutable.