Add watch script for auto recompile of Coffeescript

[anth0d]

While hacking on Dredd I like to have a watch command auto-recompiling my Coffeescript into JS

I propose adding npm run watch which basically does a build (coffee ...) with the -w flag.

[honzajavorek]

Thanks for the suggestion! Ideally, we wouldn't need this at all - see #705. However, if it helps you to hack on Dredd, then whatever works 👍 I tried it out and I think the command you want is

coffee -b -w -o lib/ src/

Usually, it's possible to add extra params to npm scripts, like this:

npm run test -- -b

That would run underlying mocha with the -b parameter. However, this isn't possible in this case, since we need to change the parameters. Adding doesn't work, I tried. So the only solution to this would be adding an extra script. I suggest build:watch to keep it in line with the other names, it would be a special version of the build command. What do you think?

@michalholasek What do you think? Would this help you to deal with CS as well?