Mini DSL to define powerline segment

[syl20bnr]

Right now we have huge monolithic blocks for the powerline. It is time to move them to the next level.

This task is about the definition of a mini DSL to define segments and compose them together easily.

The ultimate goal is to be able to define segments in package init functions and append them in arbitrary order to compose them, the render engine should be able to compute a clean powerline, i.e. it should take into account the separator directions as well as choosing the right background colors for them.

This is some work that could be contributed in upstream powerline repository.