Assets helpers

## Helpers
Implement template helpers with `Lotus::Assets::Helpers`.

Given the following template:

```erb
<!doctype HTML>
<html>
  <head>
    <title>Assets example</title>
    <%= stylesheet 'reset', 'grid', 'main' %>
  </head>

  <body>
  <!-- ... -->
  <%= javascript 'https://code.jquery.com/jquery-2.1.1.min.js', 'application' %>
  <%= javascript 'modals' %>
  </body>
</html>
```

It will output this markup.

```html
<!doctype HTML>
<html>
  <head>
    <title>Assets example</title>
    <link href="/assets/reset.css" type="text/css" rel="stylesheet">
    <link href="/assets/grid.css" type="text/css" rel="stylesheet">
    <link href="/assets/main.css" type="text/css" rel="stylesheet">
  </head>

  <body>
  <!-- ... -->
  <script src="https://code.jquery.com/jquery-2.1.1.min.js" type="text/javascript"></script>
  <script src="/assets/application.js" type="text/javascript"></script>
  <script src="/assets/modals.js" type="text/javascript"></script>
  </body>
</html>
```

Let's have a look at the corresponding Ruby code.
In this example we use ERb, but remember that `Lotus::Assets` is compatible with
all the rendering engines such as HAML, Slim, Mustache, etc..

```ruby
require 'erb'
require 'lotus/assets'
require 'lotus/assets/helpers'

class View
  include Lotus::Assets::Helpers

  def initialize
    @template = File.read('template.erb')
    @engine   = ERB.new(@template)
  end

  def render
    @engine.result(binding)
  end
end

View.new.render # => HTML markup
```

## Configurable assets

### Existing asset types

Right now the framework only supports javascripts and stylesheets.
If a developer wants to customize the output of those kind of assets he or she can do:

```ruby
require 'lotus/assets'

Lotus::Assets.configure do
  define :javascript do
    path '/js'
  end
end
```

This will generate use `/js` as output path instead of `/assets`.
While stylesheets will still use the default value. 

```html
<script src="/js/application.js" type="text/javascript"></script>
<link href="/assets/grid.css" type="text/css" rel="stylesheet">
```

### Custom asset types

The framework is so flexible that developers can define custom asset types.

```ruby
Lotus::Assets.configure do
  define :custom do
    tag %(<script src="%s" type="application/x-custom" />)
    source %(%s.cst)
  end
end
```

**Please note that this last feature is still in development, but here we have already a solid ground for it.**