convert issue markup

Jira issues use a special markup that isn't supported by redmine and seems to be a mixture of textile, markdown and some other things. Redmine seems to switch markup language to github flavored markdown starting with Redmine 5 where this feature is marked experimental.

During my experiments with issue imports, I added a conversion function to import.rb that uses pandoc to switch to github flavored markdown which is capable of converting between these two markup languages. See the attached patch. The current patch is rather sketchy and was only used to test whether this would work at all.