Add itunes genre support for podcast RSS feeds

Refer: https://podcasts.apple.com/us/genre/podcasts/id26 for full list of genres supported by <itunes:genre> tag.

MG = Main Genre
SG = Sub Genre

iTunes genre can be of the following 2 basic forms:

1. MG ( E.g. Business )
2. MG/SG ( E.g. Science/Astronomy )

Combinations of the above 2 forms are valid too:

1. MG,MG/SG etc ( E.g. Music,Leisure/Hobbies )
2. MG1/SG1,MG2/SG2 etc ( E.g. Business/Investing,Education/Self Improvement )