Possible markdownlint improvements

[mowies]

Details

Markdownlint has big potential for us, but the rules are not refined enough yet.

Some possible things to catch in addition to what is happening now. (there is a separate ticket to fix already existing rules for our usecase):

• double spaces
• spaces at the end of a line
• space before a . at the end of sentences
• double .

[StackScribe]

Can it be made to catch ".." at the end of a sentence without barfing about "..." as shorthand for "more"? That would be wonderful