Update HTML boolean attributes per the HTML 5.1 spec

A bunch of new boolean attributes have been introduced by the W3C over the past year.  This just makes Rails aware of them, so it can handle them correctly - i.e.

~~~ruby
tag("table", :sortable => true)

#=> <table sortable="sortable">

# Instead of <table sortable="true">
~~~

The new attributes:

- `allowfullscreen` (http://www.w3.org/html/wg/drafts/html/master/single-page.html#attr-iframe-allowfullscreen)
- `default` (http://www.w3.org/html/wg/drafts/html/master/single-page.html#attr-menuitem-default)
- `inert` (http://www.w3.org/html/wg/drafts/html/master/single-page.html#attr-inert)
- `sortable` (http://www.w3.org/html/wg/drafts/html/master/single-page.html#attr-table-sortable)
- `truespeed` (http://www.w3.org/html/wg/drafts/html/master/single-page.html#attr-marquee-truespeed)
- `typemustmatch` (http://www.w3.org/html/wg/drafts/html/master/single-page.html#attr-object-typemustmatch)

Also fixes the `seamless` attribute, which is currently misspelled as `seemless`.