Use built in helpers as call expression

• add Attribute values without quotes to unsupported places list in the documentation, for example the following is not supported:
  <div attribute={{# if(someTruthy)}} "foo" {{/ if}}></div>

This is part of #630

• Built in helpers can be called as nested call expression #434:
  {{# and(eq(foo, true), eq(bar, true))}} and {{else}} !and {{/ and}}