This adds the Powerline face to our set of built-in faces, which
represents glyphs that are used to draw Powerline-style shell prompts
and status lines.
Since these are used similar to box-drawing characters, alignment is
important, and this gives us the most control in preventing related
artifacts.
This initial commit is scaffolding and support for the various solid
triangles - additional glyphs will come in other commits.
Fixes#675.
