Use the `repository` to implement a "build-head` function

[wolfv]

Sometimes you want to try to build the latest and greatest software. It would be nice if we could swap out the first of the source elements to instead use git and pull from a configured repository (from the about section). We would probably also want to change the build string or version to something derived from a date or a git hash ...

brew has a functionality like this, using brew --HEAD ...

Tangentially related - if a package includes a recipe, it could be nice to add some helper functions to extract the recipe from the package (in order to use it to rebuild or modify the recipe).