feat: split long text to multiple

Google Translate limit is 100 characters,
we need to split longer text
and make multiple requests
feat: stack splitted audios

Play next utterance after all splitted audios has been played.
fix: splitted string can not be empty
chore(demo): use polyfill instead native functionality
Merge pull request #5 from janantala/feat/char-limit

Split long text into multiple requests