html2haml should be aware of comma multiline folding

[mattwildig]

(originally reported by @nex3 as haml/haml#216)

For example,

<%= foo :bar,
        :baz %>

gets multiline pipes added. This is no longer necessary.

[uekkie]

I also hope to get rid of multiline pipe.

HAML that raises an exception

index.html.erb

<%= link_to "root",
    root_path %>
<%= link_to "root",
    root_path %>

> html2haml index.html.erb

index.html.haml

= link_to "root", |
  root_path       |
= link_to "root", |
  root_path       |

This haml raises ActionView :: SyntaxErrorInTemplate.
Because the multiline pipe continues to the next element.