maintenance, (app) features, & account-features commands

I've added the command set for maintenance, app features, and account features. The styleguide that includes these changes is visible in [staging](https://hkdist-staging.herokuapp.com/styleguide.html). /cc @max 

## Maintenance mode

```
Usage: hk maintenance [-a <app>]

Maintenance shows the current maintenance mode state of an app.

Example:

  $ hk maintenance
  enabled
```

```
Usage: hk maintenance-enable [-a <app>]

Enables maintenance mode on an app.

Example:

  $ hk maintenance-enable
  Enabled maintenance mode on myapp.
```

```
Usage: hk maintenance-disable [-a <app>]

Disables maintenance mode on an app.

Example:

  $ hk maintenance-disable
  Disabled maintenance mode on myapp.
```

## Features

Originally I named these `app-features`, but I went back to `features` because none of the other app-related commands have an `app-` prefix.

```
Usage: hk features [-a <app>]

Features lists Heroku Labs features for an app.

Example:

  $ hk features
  +  preboot
     user-env-compile
  +  websockets
```

```
Usage: hk feature-enable [-a <app>] <feature>

Enables a Heroku Labs feature on an app.

Example:

  $ hk feature-enable preboot
  Enabled preboot on myapp.
```

```
Usage: hk feature-disable [-a <app>] <feature>

Disables a Heroku Labs feature on an app.

Example:

  $ hk feature-disable websockets
  Disabled websockets on myapp.
```

## Account Features

```
Usage: hk account-features

Account-features lists Heroku Labs features for your account.

Example:

  $ hk account-features
  +  pipelines
```

```
Usage: hk account-feature-enable <feature>

Enables a Heroku Labs feature on your account.

Example:

  $ hk account-feature-enable pipelines
  Enabled pipelines.
```

```
Usage: hk account-feature-disable <feature>

Disables a Heroku Labs feature on your account.

Example:

  $ hk account-feature-disable pipelines
  Disabled pipelines.
```