TOML for posts & theme

New features:

• Post's data is now stored in TOML files (instead of __main__ of package).
• Post's data path are now specified in command line;
• Output filename can be passed using --output FILENAME optional flag;
• Theme is now stored in TOML files, with pet default theme. On --theme THEME flag, if THEME is an existing file, then the file is loaded as a Theme TOML. If not, then the package searches for themes in its themes directory. Error is raised if none is found.