add right-to-left (rtl) rendering support

[fadishei]

Hello

In some languages such as Persian and Hebrew, text flows from right to left. Currently, the support for such languages by markdown editors is dismal. I have added this RTL rendering option due to my need and I think it would be useful for some others too. It adds a checkbox to the preferences screen which causes required CSS to be added to the rendered HTML.

Regards, Fadishei

[gsantner]

@fadishei Hi there, as the project is no longer maintained/dead, and not available on F-Droid too anymore, I wanted to keep it alive. For that I forked and made it compile and runable again with current sdk and appcompat. I also cherry-picked your commits of this PR. I don't know yet where it will lead, and how much time I can invest, but I want to at least keep it updated to latest Android SDK.

This is my repository: https://github.com/gsantner/marowni

[EvoHEvi]

Very good work