Use HTML's data-* attributes to support other comment types

[BubuAnabelas]

Using HTML's data-* attributes and mapping them to gettext comments could facilitate some translation processes and it should be fairly easy to implement.

According to MDN's documentation:

HTML5 is designed with extensibility in mind for data that should be associated with a particular element but need not have any defined meaning. data-* attributes allow us to store extra information on standard, semantic HTML elements without other hacks such as non-standard attributes, or extra properties on DOM.

Because of this, defining some specific data-* attributes wouldn't break anything and it could be used to implement some other features such as #4369

For example, having an HTML file like this and using html2po could make the following conversion:

<h1 data-translate-translator-comment="This is the first text that is displayed">Hello world!</h1>

# This is the first text that is displayed
msgid "Hello world!"
msgstr ""

[Frenzie]

I'd also suggest something along the lines of data-translate-ignore (assuming it doesn't already exist and is merely not mentioned in the documentation).