Added ability to use library in webbrowser with requiring stopwords files

[ageitgey]

This is a PR of a commit from @cjanietz

@cjanietz are you ok with me pulling this in?

[cjanietz]

Go ahead, fine for me

[justinmchase]

Maybe drop fs entirely and convert the stop word files to code:

# en.coffee
module.exports = [
  "a's",
  "able",
  # ... etc.
]

stopwords = require(language)