Version v3.1.0

This PR enhances the `cyr2lat` and `lat2cyr` CLI tools by:

- Allowing an optional second argument to specify the output file.

- Automatically generating the output filename if not provided (e.g., input.txt → input-lat.txt or input-cyr.txt).

- Displaying a helpful usage message when no arguments are passed.

```shell

cyr2lat input.txt               # Output: input-lat.txt

lat2cyr input.txt output.txt    # Output: output.txt

cyr2lat                         # Shows usage message

```