Add publicPath to build-javascript and build-html

[tribou]

Use linkPrefix for publicPath when set
Fixes #392

Unfortunately, this doesn't work because I mistakenly thought gatsby was using the config package to pull in the config.toml. Is a project's config.toml already available to the gatsby cli context somewhere?

[KyleAMathews]

Hmm... yeah, another indication that config.toml was a bad idea. My plan is to switch things over to a gatsby.config.js soonish anyways as we'll need that for theme/plugin support so perhaps shelf this until then?