Reduce intrusiveness of :target styling

Turning the entire method background yellow was overly intrusive and
confusing if you happened to hover over another method.  Now a border
appears on the left hand side of the method to indicate what you clicked
on.
Add font-family fallbacks for monospace
Do not update class position for class << self

After several refactorings this bug was missed due to insufficient
tests.

When parsing "class << self" the position of the class should not be
updated as we're reopening the same class as a singleton.  Instead, just
return from parsing the class.
Fix handling of private

After refactorings this bug remained due to insufficient tests.

When setting the visibility of a method, include the singleton flag to
set visibility on the correct method.  This fixes a crash.
Reduce ragged left edge

This pushes content elements to the left except for lists and
preformatted text.  Suggestion from @veganstraightedge.