Don't change default encoding globals

[jrochkind]

Hi, I'm looking into existing ruby code for dealing with webvtt, thanks for making this available!

Loading this gem causes the global default encodings for ruby to be changed:

webvtt-ruby/lib/webvtt.rb

Lines 3 to 5 in f3c1403

	if defined?(Encoding)
	Encoding.default_internal = Encoding.default_external = "UTF-8"
	end

An app may have it's own reasons for choosing different default encodings. It's not polite to silently change them after loading a gem.

Is it possible to make the webvtt-ruby code use explicit encodings in it's calls where it wants them, instead of changing the global defaults?