Add optional friendly-key name argument, pull text from [scriptname.js]

Not all strings make good key names (e.g. those with control characters),
so allow an additional, optional argument to gettext() that allows for
more friendly key names to be used to specify/look-up (some) text strings.

Also, in addition to the global [js] section for these text values, look
(first) in [scriptfilename.js] - this allows more customization on a
per-script basis (e.g. [default.js] can be used to override strings used
in the Synchronet Classic command shell (default.js) but would not effect
if/when those same strings are used in other scripts/shells.