Support non-breaking inline characters

[aidantwoods]

• Non breaking spaces, possibly escaped space \• or double space •• (would possibly require having this independent of hard line breaks, or at least some priority to be established).

• Non breaking hyphens (possibly just default this for intraword hyphens) .

Existing discussions:

• https://talk.commonmark.org/t/use-escaped-space-as-nbsp/1907/3
• non breaking spaces and non breaking hyphens Orc/discount#119