Update HTML keywords (#1530)

Update HTML keywords (#1530)

* Add newly standardized HTML elements to filetypes.html

To enable highlighting them as keywords in HTML documents, add
the following elements defined in W3C HTML 5.1
(https://www.w3.org/TR/html51/fullindex.html#index-elements):

- menuitem
- picture
- rb
- rtc
- template

as well as the following elements defined in WHATWG HTML Living Standard
(https://html.spec.whatwg.org/multipage/indices.html#elements-3):

- math
- slot
- svg

* Add newly standardized HTML attributes to filetypes.html

To enable highlighting them as keywords in HTML documents, add
30 attributes defined in W3C HTML 5.1
(https://www.w3.org/TR/html51/fullindex.html#attributes-table)
and a further 15 attributes defined in the WHATWG HTML Living Standard
(https://html.spec.whatwg.org/multipage/indices.html#attributes-3).

* Sort the list of HTML keywords