Better support for transposition and Nashville numbers.

- Non-diatonic pitches are supported using Nashville numbers (#5).
- When using Nashville numbers chord variants with non-diatonic roots
  are now recognized (e.g. b7)
- Pitch rendering is now aware of key centers. For example, F# is F#
  rendered as F# when in the key of G but Gb when in the key of Ab.

src/pco_chordspkg/cliconstants.nim

@@ -1,4 +1,4 @@
-const PCO_CHORDS_VERSION* = "0.5.0"
+const PCO_CHORDS_VERSION* = "0.5.1"
 
 const USAGE* = """Usage:
   pco_chords [options]