Installer: don't add main in existing projects

[shafy]

This addresses and closes #139.

Currently, the installer adds a <main> element around <%= yield %> in application.html.erb. This is ok for new projects, but it's annoying for projects that add this gem later on since they likely already have changed their application.html.erb and installing the gem breaks the layout.

This proposed change first checks if application.html.erb looks standard around <%= yield %>, and only in that case wraps it in a <main> element.

[dixpac]

I'm ok with this approach. But @dhh has the last word.