Allow for appending for strings without HTML

Currently, calling `downsize("A string without HTML", {words: 2,
append: '...'}` returns `A string`.

I changed `(stack.length || tagBuffer.length)` to `isAtLimit()`, which
solves this issue and makes that line a little more clear.

Tests included.
Merge pull request #19 from stevehansell/append_without_html

Allow for appending on strings without HTML