Upon running `'w3m-session-backup`, this error message would be printed:

    Symbol's function definition is void: string-join

I learned that Emacs doesn't load all functions at load. In particular
for this case, the `string-join` function was in a file not loaded by
default.

Thanks to the following resources:
https://emacs.stackexchange.com/questions/22142/cannot-open-load-file-subr-x
https://github.com/ralesi/ranger.el/blob/9db73d61/ranger.el#L84

I discovered I could require the `subr-x.el` file that contains the
`string-join` function needed. Decided to keep using `string-join`
instead of `(mapconcat 'identity ...` as it reads better in my mind.

Instead of printing a generic success message, include the filename and
path of the newly saved backup file.

Extract the code that generates the file path from
`w3m-session-backup--save-backup` to a new function so it can be reused.

Provide some user confirmation of the backup action. Otherwise, you're
left in doubt about whether the command actually ran.

Now that the package is published on GitHub, use that as the URL.