File format conversions using Calibre CLI

[Shimmermare]

Use Calibre CLI (https://manual.calibre-ebook.com/generated/en/ebook-convert.html) to convert story files to different formats.

Requirements:

• Detect if Calibre is installed, and if not - offer official download page link.
• Add all supported by Calibre formats as supported by app.
• Add generic way to count words using conversion to TXT.