Add global cli flag (ref #692)

Adds a --global cli flag to define global variables. To define
multiple variables, either separate them using commas, or use the
flag multiple times. By default, all variables are read-only, but
appending `:true` to the name will mark it as writable.

Example:

    eslint --global require,exports:true file.js
    eslint --global require --global exports:true file.js