Add stars support

This is a work in progress.

### apm stars

List or install starred packages/thems by yourself or someone else.
```
$ apm stars -u thedaniel
Packages starred by thedaniel (5)
├──  autocomplete-emojis Adds emoji autocompletion to autocomplete+ (110 downloads)
├── block-travel Quickly travel up/down between blocks of code (1133 downloads)
├── color-picker A Color Picker for the Atom Editor. Right click a color and select color picker to open it. (34316 downloads)
├── linter Validates your code using linters. (14454 downloads)
└── pomodoro Simple pomodoro timer (81 downloads)

Use `apm install` to install them or visit http://atom.io/packages to read more about them.
```