feature, allow for different dummy text libraries

[paulcollett]

Possible interfaces:
<script src="https://dummyjs.com/dummy.js?lib=bacon"></script>
<script src="https://dummyjs.com/dummy.js" data-lib="bacon"></script>

Allowing external libraries:
<script src="https://dummyjs.com/dummy.js" data-lib="http://bacon.gen/bacon.js"></script>

Providing custom text:
<script>window.dummyjs_lib = 'custom text lipsum bacon';</script>
<script>window.dummyjs_lib = { words: 'custom text lipsum bacon', join: 'in on' };</script>