Transposing "slash chords" is problematic

[saebekassebil]

Issue is described in #83 as well.

Transposing chord Amaj7/C# a minor third should produce Cmaj7/E but right now it returns Cmaj7/C# which is clearly not correct.

The problem is that d'accord is for "note agnostic" and is more concerned with translating chord symbols into an array of intervals.