Allow elements of columns to be simple hyperlinks

[Datseris]

Hi, I would like to have some elements of the columns in the top multi-documenter bar to be just normal hyperlinks. So, e.g.,

clicking this should lead to a pre-described hyperlink instead of including and deploying the whole gh-pages stack of the named package.

This is particularly useful in e.g.,

1. Packages whose documentation is the Readme.
2. Packages that are also based on multidocumenter such as DifferentialEquations.jl.

[pfitzseb]

Fixed by #86.