Support for custom module paths for jshint/jslint

Alternatively to the bundled jshint.js and jslint.js, the global
`jshint` and `jslint` modules can be defined by setting
`jshint-mode-jshint-path` or `jshint-mode-jshint-path` in emacs
configuration.

Eg: For the global installation of jshint, set
`jshint-mode-jshint-path` to "jshint"

    (setq jshint-mode-jshint-path "jshint")

Ref issue: #16