added /series command

[dashroshan]

Added new command/series

• [x] Code is formatted correctly
• [x] Code is tested before submitting
• [x] Added command to README.md commands list

Details

The user enters the name of the series to lookup using the /series command and the bot shows an embed with the series name, description, cover image, rating, status, previous and next episode name and release dates.

This is in reference to this issue