Fix multiline docstrings

Commit e66a40683e2ebf94a1e3bea53706c55ab879c7cd fixed unintentional
unescaping of docstrings, but also broke multiline docstrings:

        (ns example
          "Foo
           Bar
           Baz")

        =>

        (ns example
          "Foo\n   Bar\n   Baz")

This approach manually escapes \ and " inside of docstrings, but leaves
newlines and other characters as is so that the string is reprinted as
intended.
Merge pull request #49 from guns/fix-ns-multiline-docstrings

Fix multiline docstrings