Add methods useful for status lines

I added three functions and corresponding methods that are useful for plugins like airline or powerline.

- `commandt#CommandTActiveFinder` should be used to determine whether path is to be shown and what kind of selection is being made.
- `commandt#CommandTPath` should be used to display a directory from which file finder is selecting a path.
- `commandt#CommandTCheckBuffer` should be used to determine whether Command-T specific stuff should be displayed at all.