Refactored kepubify command, and improved docs

docs/index.html

@@ -91,18 +91,25 @@
 USAGE: kepubify INPUT_PATH [OUTPUT_PATH]
 
 INPUT_PATH:
-    The input file or directory.
-
- OUTPUT_PATH:
-    The path to place the converted ebook(s). This path must not exist
-    already.
-
-    By default, for a file input, this is the basename of the input file, 
-    with the extension .kepub.epub. For a directory input, this is the 
-    directory name, with the suffix _converted.
-
- The full documentation is available at:
-    https://geek1011.github.io/kepubify</pre>
+   The input file or directory.
+
+   For single converting, this must be an existing file with
+   the extension .epub.
+
+   For batch converting, this must be a existing directory.
+
+OUTPUT_PATH:
+   The path to place the converted ebook(s).
+
+   If single converting, this path will be overwritten if it exists. It
+   must also end with .kepub.epub.
+
+   If batch converting, this path must not exist, an will be automatically
+   created.
+
+   If not specified, then for a single converting, this is the basename
+   of the input file with the extension .kepub.epub. For batch converting,
+   this is the directory name, with the suffix _converted.</pre>
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