Support Elm binaries in project-local NPM packages

[sentience]

Elm Language Support relies on binaries (elm-make, elm-oracle, etc.) installed globally on the system.

It is possible (though not officially supported) to install the Elm platform in a project's local node_modules directory. @bholtbholt has requested in #12 that Elm Language Support detect and use these locally-installed binaries when they are available.

See the README for the elm npm package for the official installation instructions, which say to install the package globally.