Add source code possibility for RTF output

Analogous to Latex and HTML output the possibility to have source code shown in RTF output is created.
Based on question: http://doxygen.10944.n7.nabble.com/RTF-lack-of-source-code-tc6936.html
Merge pull request #264 from albert-github/feature/bug_rtf_source

Add source code possibility for RTF output