Increase minification: emit_attributes() should pick single or double quotation form for escaping

[raphaelcohn]

A nice feature to improve minification would to let emit_attributes() choose to use either a single or double quoted form for attribute values depending on the presence of double or single quotes in the attribute value to be escaped. This obviously has a performance hit, so it should be an option in EmitterConfig.

[netvl]

Good idea! Pull requests are of course welcome. Not sure if I will be able to work on this any time soon.