Stripping whitespace around sentences is destructive

[jordan-smith]

As mentioned in issue https://github.com/wikimedia/sentencex/issues/10 sentencex strips whitespace around sentences so it's not really "non-destructive" like the docs state. It can be shown in the demo: if you add multiple whitespaces at the beginning of a sentence, they don't appear in the output.

While it's easy to restore the missing whitespace to the output by comparing it to the full text, it would be nice if sentencex wouldn't strip it in the first place, or if that were optional. At the very least the docs should be corrected in the meantime to avoid confusion.