CLI now accepts strings as stdin

[AucaCoyan]

Finally I modified clap to accept either a file or a string.

It came with a couple of architectural changes:

• ➖ I decided to clean the Session struct, because it wasn't really relevant to have it.
• ➖ Input was also a rock the way to interpret files to format. So also it was deleted for better code readability
• ➕ I added 2 functions: format_single_file and format_string that redirects the code to format_inner, where the code is formatted. The first 2 functions have the responsibility of reading & writing the file, or writing to stdout respectably.

[fdncred]

Looking good! Thanks again!