corner-case for ''' strings

[florianschanda]

In the LRM we have something like this:

  bnf = '''
    foo ::= bar
            { wibble }
  '''

This comes out as

foo ::= bar
{ wibble }

It would be better if it came out like this:

foo ::= bar
        { wibble }

We could change the rules to only ignore absolute first line (i.e. the stuff on the same line as the ''' when figuring out common whitespace). But still ignore blank lines. But this is rapidly becoming a mess.