Tag Helpers: `<option/>` tag helper

[dougbu]

Current <select/> tag helper only sets selected attributes for <option/> elements generated from the items enumeration. Should support adding this attribute to <option/> elements users provide. E.g. f the Razor source contains

<select for="YearsEmployeed">
  <option value="">Why didn't you select anything?</option>
  <option value="0">Less than 1</option>
  <option value="1">1</option>
  <option value="2">2</option>
  <option value="3">3</option>
</select>

the generated code should select the <option value="1"> element when Model.YearsEmployed is 1.

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As an added bonus, could perform no automatic selection if the user explicitly provides a selected attribute on one of their options. E.g. if the Razor source contains

<select for="YearsEmployeed" items="SelectList">
  <option value="">Why didn't you select anything?</option>
  <option value="0">Less than 1</option>
  <option value="1">1</option>
  <option value="2" selected="selected">2</option>
  <option value="3">3</option>
</select>

the generated code should include only a single selected attribute even if SelectList contains a SelectListItem for 4 and that's the current Model.YearsEmployed value.
If we decide to support this enhancement, it may need to be tracked as a separate issue.

[dougbu]

Commit 2d32420 with support from preceding commits 3d84b52 and 30f25fe.