Unicode whitespace is stripped from leading and trailing positions in markdown paragraphs

[tim-evans]

Following up on #307, we should handle a longer list of unicode whitespace characters:

Name	Code Point	Entity	Size
No Break Space	\u00A0	 	👉 👈
En Quad	\u2000	 	👉 👈
Em Quad	\u2001	 	👉 👈
En Space	\u2002	 	👉 👈
Em Space	\u2003	 	👉 👈
Thick Space	\u2004	 	👉 👈
Mid Space	\u2005	 	👉 👈
Six-per-em Space	\u2006	 	👉 👈
Figure Space	\u2007	 	👉 👈
Punctuation Space	\u2008	 	👉 👈
Thin Space	\u2009	 	👉 👈
Hair Space	\u200A	 	👉 👈
Zero Width Space	\u200B	​	👉​👈
Narrow No-break Space	\u202F	 	👉 👈
Medium Mathematical Space	\u205F	 	👉 👈
Ideographic Space	\u3000	　	👉 👈
Zero Width No-break Space	\uFEFF		👉👈

I think this is a fairly exhaustive list of spaces, but if any more should be added, please comment 😄