Support i18n on javascript files

UPDATE

This issue has been closed a "won't fix", as it's been surpassed by #246 (closed)

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Currently only files that match the pattern *.i18n.html are supported by the i18n engine. Such support should be extended to other text based files as well (for now only .js files come to my mind, so maybe we should stick to html and js)

For such files, the translation placeholder should keep {{translate "text"}} as in the html pages, although ``` should be be used in case the placeholder is inside quoted text (just as a note, as that's already implemented).

One issue we'll face is having to rename all references to '.js' files to '.i18n.js' whenever they are used, as such name is required for them to be dynamically translated.