fixes #57, #46

- add .editorconfig file
- format all files to comply with castopod's coding style
- switch parsedown dependency with commonmark library to better follow commonmark spec for markdown
- add prettier command to format all project files at once

closes #16

- refactor episode, podcast and category entities to add dynamic properties
- refactor Routes when adding feed route
- update migration files to better fit itunes' and rss' specs
- update podcast and episode forms
- add SimpleRSSElement class to Libraries
- add rss_helper
- update home controller to redirect if system has only one podcast