Support the + register as an alias of the * register (system clipboard)
Add % register (current file path)
Implement blackhole register
Map the rest of the lower case registers
Update docs/overview.md to expand on what works with Registers
Adapt code style with suggestion from pull request
Flesh out register specs

Now it covers:
  a
  +
  *
  _

I have a pending spec for %, since I'm not sure how to setup the editor
variable to behave correctly.
Property stub editor.getUri so we can spec the % register
Flesh out comment about the + register needing a Linux update in the future

- Specifically, + and * work the same everywhere except X11 based
  environments, so just comment it until we have a linux build and can
  deal with it correctly
Merge pull request #127 from cschneid/register_improvements

Register improvements