Whitespace at the beginning or end of bold region of text breaks parsing

[GabeIB]

Text like this **whitespace at the end of bold region ** and this ** whatespace at the beginning of bold region** are not recognized as text that should be made bold by the blackfriday parser.

While I expected **bold text ** to be rendered to something like this: <b>bold text </b>. It is instead just treated as text.

While, github's markdown renderer has the same behavior, it might be nice to make blackfriday more resilient to user error.