Add option to add arbitrary attributes to the generated <script> tag

[vpavkin]

For example, it can be useful to add defer/async attribute to optimize page load time.

[vmunier]

Hi,

We could add an additional htmlAttributes: Html parameter with a default value, so that the new library version remains compatible with existing code.

Parameters of src/main/twirl/scalajs/scripts.scala.html would look like:

@(projectName:    String,
  assets:         String => String,
  resourceExists: String => Boolean
  resourceExists: String => Boolean,
  htmlAttributes: Html = Html("")
)

Usage would be:

@scalajs.html.scripts(
  projectName = "client",
  name => s"/assets/$name",
  name => getClass.getResource(s"/public/$name") != null,
  Html("defer async newAttr=newValue")
)

What do you think?

[vpavkin]

looks perfectly fine to me :) Sorry, haven't had time to make a pr yet :(

[vmunier]

No worries, I'll include the changes.