Publishing

In order that urls work properly on the published site, you can pass in a custom url root path via the `MEGPHYS_ROOT` environment variable. If you don't set a value it defaults to `/`.
I've also added a new option to `megphys`: `publish`.
`./megphys publish` will build the site with the correct `MEGPHYS_ROOT` value for GitHub Pages, copy the contents of the build folder, checkout `gh-pages` and paste the contents in the main folder. All you need to do then is commit the changes and push to the remote `gh-pages` branch.