Add textile issue template

[felixbuenemann]

In plugin release v0.4.0 / PR #2 the built-in textile template was replaced with errbit's default markdown template, but this leads to wrong display, if Redmine is configured to use Textile instead of Markdown as it's markup language.

Historically Redmine has used Textile, but newer versions have switched to Markdown.

This is a global setting, so the plugin should be configurable to create issues using either the textile or markdown format.

According to errbit/errbit#840 an issue tracker plugin can implement render_body_args to return the arguments to Rails' render_to_string method, eg. ['issue_trackers/issue', formats: [:md]].

I'm currently trying to find out how view paths from the plugin are supposed to be added to the errbit apps view lookup context, which would be required if the template is stored in the plugin as opposed to errbit. Another idea would be to submit a default textile template to errbit and only switch the format in the plugin.

[stevecrozz]

@felixbuenemann One idea which I think I've tested long ago is to not add any new view paths, but instead to supply the full path to the template as the first argument in render_to_string.

[felixbuenemann]

That's worth a try. You can do render file: "/some/absolute/path" so it should work with render_to_string as well.