Partial fix for #1 - the download url now works

But the output filename is wrong - it is <slug> rather than <id>.json
Add workaround for #1 - commands to rename files

Append the .bat or .sh commands that the user can execute, to rename
the saved files, so that they are named after their board_id, not the
8-character
"slug"
Finished workaround for #1 - added instructions

The output .html file now contains text telling the user
how to use the generated links
Added a credit, and URL of this project, for info
Added error message, if run on non-Trello site
Add link to an alternative (api-based) approach