Allow serving static files with GHCJSi

[fizruk]

This changes allows to serve static files from GHCJSi browser session and makes it possible to:

• run apps with images, CSS files and other stuff from GHCJSi;
• have custom ghcjsiClient.html and/or ghcjsiClient.js.

This relies on finalhandler and serve-static packages which one can install using npm:

npm install finalhandler serve-static

I'm not savvy with nam packages, and there seem to be more alternatives to static file servers.
If you think different packages should be used — feel free to change that.

By default static server is disabled. To serve files set GHCJSI_STATIC_DIR environment variable, e.g:

export GHCJSI_STATIC_DIR=$(pwd)/static

A gist with example usage: https://gist.github.com/fizruk/fb5bad8a08a4b788764851c11ebbadf8

[dmjio]

Bravo @fizruk :) not afraid to write js!

[fizruk]

I've just uploaded a Docker image fizruk/stack-ghcjs with support for static files built in.
Hopefully this will be useful to someone besides me :)