web: add a --static-root option to customize static files root

Right now hledger-web serves its static files using Static.embed for non-development build. One problem with Static.embed is that it doesn't output any headers for browsers to cache them. Every time a register page loads, around 92KB of static resources are re-loaded from server. This new option enables the optimization that the static files can be served from a generic web server like apache, which is good at handling static files and caching. One can also serve the files in a separate domain to reduce cookies overhead.