Add stars support

Closes #117 

### 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.
```

```
$ apm stars --install
Installing hex to /Users/kevin/.atom/packages ✓
```

### apm star

```
$ apm star wrap-guide command-logger toggle-quotes
Starring wrap-guide ✓
Starring command-logger ✓
Starring toggle-quotes ✓
```

### apm unstar

```
$ apm unstar toggle-quotes
Unstarring toggle-quotes ✓
```