Previously, we'd fail if files had no numbers. Now this ignores and
continues anyway.

- Responds to next / previous track events
- Sets metadata for the currently-playing track

This moves the minimum requirements to Go 1.16 to build this project.

plus set charset = UTF-8

This allows the user-defined mixtape.tmpl to define track notes for each
individual track. By default, it only shows notes for the
currently-playing track.

This also logs the address the application is serving on.

if there is no mixtape.tmpl in the destination directory.

Run `cdr burn` to create an index.html file in the current directory.

This serves the mixtape site by running `cdr preview`

This adds a NewTrack func that creates the struct, and we now use this in RenameTrack().

Instead of taking a filename as an argument, we now change all files in the current directory.