Ability to use dynamic updates

On the python side
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Just use the standard string expansion methods like _("This %s is expanded" % "String").

On the javascript side
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Updated the _-translate method to accept a second replacement parameter like _("This %s is expanded", ["String"]). The (already included) sprintf for Javascript is used to do the replacement.

Translation extraction
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The regexps in the method used for translation-string-extraction is extended to fit the possibilities.

[Issue addressed in Issue #859 of the erpnext-repository](https://github.com/webnotes/erpnext/issues/859)