Analyze Ruby code in ERB interpolated HTML attributes

[marcoroth]

Fixes #68

This works now:

ERB Input:

<div class="<%= @hello %>">Hello</div>

Output:

class HelloComponent < Phlex::HTML
  def initialize(hello:)
    @hello = hello
  end

  def template
    div(class: @hello) { "Hello" }
  end
end