Add support for services

[lee-dohm]

With atom/atom#5165 and atom/atom#5277 merged, we should have a method of documenting services provided akin to events.

I'm thinking something along the lines of:

## Service `status-bar` v0.57.0

### `consumeStatusBar`

Essential: Provides a reference to the status bar to the consuming package.

* `statusBar` Status bar {Object}
    * `addLeftTile` A {Function} for adding a tile to the left side of the status bar
    * `addRightTile` A {Function} for adding a tile to the right side of the status bar
    * `getLeftTiles` A {Function} for getting the list of tiles on the left.
    * `getRightTiles` A {Function} for getting the list of tiles on the right.