Add support for configuring path to istanbul executable

In my Rails project, istanbul is packaged as a Node dependency. The path to the executable is then `./node_modules/.bin/istanbul`.

Some devs on the team have this path in their shell `PATH` var, but some do not. As there are some breaking API changes with the latest istanbul (0.3.0), it would be nice to tell Teaspoon to use the version bundled with the app and not bother searching the system path.

I wouldn't imagine this would be terribly difficult, would it?