Add linting for Markdown prose

Codify existing practices for writing Markdown in style guides and
enforce them via Markdownlint. A new npm script "lint" in the top level
package.json runs before tests or as the first step of the "travis"
script.

Only modify documents in cases where they had bugs or isolated cases of
inconsistency:

README.md: 10: MD007 Unordered list indentation
Inconsistent with all other top level lists

README.md: 10: MD032 Lists should be surrounded by blank lines
Some Markdown parsers don't handle this correctly

README.md: 3156-3161: MD005 Inconsistent indentation for list items at
                      the same level
Bug, looks like it's intended to be another list level but GitHub
renders it at the same level as the "No but seriously"

README.md & css-in-javascript/README.md: throughout: MD012 Multiple
          consecutive blank lines

README.md: throughout: MD004 Unordered list style
Some nested lists used plusses, now everything consistently uses
dashes.

css-in-javascript/README.md

@@ -176,7 +176,6 @@
 
     export default withStyles(() => styles)(MyComponent);
 
-
     // good
     function MyComponent({ styles }) {
       return (