exp/template/html: Grammar rules for HTML comments and special tags.

exp/template/html: Grammar rules for HTML comments and special tags.

Augments type context and adds grammatical rules to handle special HTML constructs:
    <!-- comments -->
    <script>raw text</script>
    <textarea>no tags here</textarea>

This CL does not elide comment content.  I recommend we do that but
have not done it in this CL.

I used a codesearch tool over a codebase in another template language.

Based on the below I think we should definitely recognize
  <script>, <style>, <textarea>, and <title>
as each of these appears frequently enough that there are few
template using apps that do not use most of them.

Of the other special tags,
  <xmp>, <noscript>
are used but infrequently, and
  <noframe> and friend, <listing>
do not appear at all.

We could support <xmp> even though it is obsolete in HTML5
because we already have the machinery, but I suggest we do not
support noscript since it is a normal tag in some browser
configurations.

I suggest recognizing and eliding <!-- comments -->
(but not escaping text spans) as they are widely used to
embed comments in template source.  Not eliding them increases
the size of content sent over the network, and risks leaking
code and project internal details.
The template language I tested elides them so there are
no instance of IE conditional compilation directives in the
codebase but that could be a source of confusion.

The codesearch does the equivalent of
$ find . -name \*.file-extension \
  | perl -ne 'print "\L$1\n" while s@<([a-z][a-z0-9])@@i' \
  | sort | uniq -c | sort

The 5 uses of <plaintext> seem to be in tricky code and can be ignored.
The 2 uses of <xmp> appear in the same tricky code and can be ignored.
I also ignored end tags to avoid biasing against unary
elements and threw out some nonsense names since since the
long tail is dominated by uses of < as a comparison operator
in the template languages expression language.

I have added asterisks next to abnormal elements.

  26765 div
   7432 span
   7414 td
   4233 a
   3730 tr
   3238 input
   2102 br
   1756 li
   1755 img
   1674 table
   1388 p
   1311 th
   1064 option
    992 b
    891 label
    714 script *
    519 ul
    446 tbody
    412 button
    381 form
    377 h2
    358 select
    353 strong
    318 h3
    314 body
    303 html
    266 link
    262 textarea *
    261 head
    258 meta
    225 title *
    189 h1
    176 col
    156 style *
    151 hr
    119 iframe
    103 h4
    101 pre
    100 dt
     98 thead
     90 dd
     83 map
     80 i
     69 object
     66 ol
     65 em
     60 param
     60 font
     57 fieldset
     51 string
     51 field
     51 center
     44 bidi
     37 kbd
     35 legend
     30 nobr
     29 dl
     28 var
     26 small
     21 cite
     21 base
     20 embed
     19 colgroup
     12 u
     12 canvas
     10 sup
     10 rect
     10 optgroup
     10 noscript *
      9 wbr
      9 blockquote
      8 tfoot
      8 code
      8 caption
      8 abbr
      7 msg
      6 tt
      6 text
      6 h5
      5 svg
      5 plaintext *
      5 article
      4 shortquote
      4 number
      4 menu
      4 ins
      3 progress
      3 header
      3 content
      3 bool
      3 audio
      3 attribute
      3 acronym
      2 xmp *
      2 overwrite
      2 objects
      2 nobreak
      2 metadata
      2 description
      2 datasource
      2 category
      2 action

R=nigeltao
CC=golang-dev
https://golang.org/cl/4964045