feat(autocomplete): add support for aria-describedby

[cai-feng]

Adding aria-describedby to md-autocomplete like following code won't work.

<form name="projectForm">
  <md-autocomplete md-input-id="input-project-name"
                  md-input-name="projectName"
                  md-no-asterisk
                  md-floating-label="Project name"
                  md-items="item in $ctrl.projects"
                  md-item-text="item.name"
                  md-selected-item="$ctrl.project"
                  md-select-on-match="true"
                  md-autofocus
                  required
                  aria-describedby="project-name-description"
                  flex>
  <md-item-template>
      <span>
        {{item.name}}
      </span>
  </md-item-template>
  <div ng-messages="projectForm.projectName.$error"
        ng-if="projectForm.projectName.$touched">
    <div ng-message="required">
      Project name is required.
    </div>
  </div>
  <div id="project-name-description">
    Descriptive text
  </div>
  </md-autocomplete>
</form>

[Splaktar]

@cai-feng I would like to look at fixing this, but I'm not clear on what isn't working for you.

• Is the issue related to a specific screen reader's performance?
• Is the desired outcome that the aria-describedby setting get transferred to the autocomplete's input element?
• How can I verify that changes I make fix this issue?

[cai-feng]

@Splaktar Sorry for confusing and replying so late.

• This issue should impact many screen readers (ChromeVox, VoiceOver and JAWS).
• Yes. Transferring aria-describedby from md-autocomplete to its input makes sense to its users.
• Keep you screen reader open and focus on the autocomplete by moving from other element.
  Ideally, you will hear something like this {content of md-floating-label} Collapsed Required Combo box { content of aria-describedby}

I also made a demo for you. thanks.
https://stackblitz.com/edit/angularjs-gx57rp