Support `dwi` file format

`Song` structs should be readable from and writable to the `dwi` format.

`dwi` predates `sm`, and is still supported by Stepmania. Format specification is available here: http://dwi.ddruk.com/readme.php

There is also a largely-incomplete write-up about the format on the Stepmania wiki: https://github.com/stepmania/stepmania/wiki/dwi

Required Tags:

- [ ] `#TITLE`
- [ ] `#ARTIST`
- [ ] `#GAP`
- [ ] `#BPM`

Optional tags:
- [ ] `#DISPLAYTITLE`
- [ ] `#DISPLAYARTIST`
- [ ] `#DISPLAYBPM`
- [ ] `#FILE`
- [ ] `#MD5`
- [ ] `#FREEZE`
- [ ] `#CHANGEBPM`
- [ ] `#STATUS`
- [ ] `#GENRE`
- [ ] `#CDTITLE`
- [ ] `#SAMPLESTART`
- [ ] `#SAMPLELENGTH`
- [ ] `#RANDSEED`
- [ ] `#RANDSTART`
- [ ] `#RANDFOLDER`
- [ ] `#RANDLIST`
- [ ] `#SINGLE`
- [ ] `#DOUBLE`
- [ ] `#COUPLE`
- [ ] `#SOLO`
- [ ] `#BACKGROUND`
- [ ] `#END`