Stricter template validation and executor refactor

A refactor of executors, with stricter template validation.

Breaking Changes

• A template will be rejected with an error if it does not contain a template or block declaration
  for all defined sub-view names.

    tErr.Error()
    => template example.html.tmpl: missing template declaration(s) for sub view blocks: "my-block"
  

  • The above can be corrected by adding {{ template "my-block" .MyBlock }} to example.html.tmpl.

Added

• New treetop.ServeClientLibrary handler serving an embedded copy of the treetop-client JS, for convenience sake

• Add view.HasSubView method to assert the existence of a sub view name without specifying a template.

    myPage := treetop.NewView("base.html.tmpl", BaseHandler)
    myPage.HasSubView("sidebar") // no template or handler specified
  

Housekeeping

• Add test coverage for DeveloperExecutor forced reloading
• Major refactor of Executors to remove duplicate code. The behavior of all executors should remain the same!
• Replace deprecated use of HeaderMap in favor of .Header()