Adding support for heredocs used in functions.

Previously using a heredoc in a function would result in the rest of the
line after a heredoc assumed as being part of the string and would be
makred as part of the heredoc, but the heredoc doesn't start until the next
line.

  eg.  exec_sql(<<SQL, 1, 'book')
    SELECT * FROM products WHERE id = ? OR name = ?;
  SQL

This commit marks the heredoc declarations and the heredoc as part of
the heredoc, but leaves the rest of the line marked as ruby.

Also adding support heredocs with backticks.