[WIP] Disable specific applications

The implementation for #85 .
I want to change like this.

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### Disabling the plugin

The plugin can be totally skipped by setting `config.proxy.enabled` to `false` or empty string (`""`).
This can be useful to for example disable it for some provider.
Specific applications can be skipped by setting `config.proxy.enabled` to
a hash( like `{ svn: false }`).
This disabling keeps proxy configurations for applications on the guest.
The configurations must be cleared before disabling if needed.

```ruby
config.proxy.enabled         # => all applications enabled(default)
config.proxy.enabled = true  # => all applications enabled
config.proxy.enabled = { svn: false, docker: false }
                             # => specific applications disabled
config.proxy.enabled = ""    # => all applications disabled
config.proxy.enabled = false # => all applications disabled
```
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Todos: 
- [X] README
- [x] spec
- [x] implementation
- [ ] test