Keep indent level in examples

[miyagawa]

If you have a tomdoc like:

Examples

  def foo
    'bar'
  end

tomdoc.examples[0] would become

"def foo\n    'bar'\n  end"

which has a different level of indentation for the 1st line and the rest of the code.

This patch addresses the issue by reuseing the indent removal code in #tomdoc and strips the prefixed indent for each line and then recombine, so that the result will have the same indent leval you can run through as a code snippet.