Allow customizing the checkboxes in eww

Allow customizing the checkboxes in eww
* net/shr.el (shr-tag-table): Fix comment typo.

* net/eww.el (eww-text-input-types): : New const.
(eww-process-text-input): Treat input types in
`eww-text-input-types' as text.

* net/shr.el (shr-tag-video): Display content for video if no
poster is available.
(shr-tag-audio): Add support for <audio> tag.

* net/eww.el (eww-use-external-browser-for-content-type): New
variable.
(eww-render): Handle `eww-use-external-browser-for-content-type'.
Use \\` to match beginning of string instead of ^.
(eww-browse-with-external-browser): Provide optional URL parameter.
(eww-render): Set `eww-current-title' back to "".

* net/shr.el (shr-prefer-media-type-alist): : New customizable
variable.
(shr--get-media-pref): New function.
(shr--extract-best-source): New function.
(shr-tag-video, shr-tag-audio):  Use `shr--extract-best-source' when
no :src tag was specified.

* net/eww.el (eww-form-checkbox-selected-symbol): New
customizable variable.
(eww-form-checkbox-symbol): New customizable variable.
(eww-form-checkbox): Use `eww-form-checkbox-selected-symbol' and
`eww-form-checkbox-symbol'.
(eww-toggle-checkbox): Use `eww-form-checkbox-selected-symbol' and
`eww-form-checkbox-symbol'.